 to be more like, "Work as many or as few hours as you want as long as you get your work done."

This is true, but you say that he does not get his job done. Frankly, if he only works like 25 hours, even if he would be supremely productive I would still talk to him about his work ethics if he puts in so much less hours than his peers... and if he is not it's all the more reason to make sure that he at least tries as hard as the others.
If you don't, the problem is going to be that sooner rather than later many of the 45-hours students are going to wonder why the heck they are actually not enjoying the summer. I have seen in my current group that various bad habits have a tendency to spread like wildfire if the students have the impression that not doing your job isn't in any way frowned upon anyway.

If so, how to do it without sounding like a jerk?

In private, without getting emotional or accusatory. Tell him that you have the impression that he works considerably below the amount that was agreed upon, and that you have to ask him to change this. Tell him that you have the impression that his project is not progressing fast enough, and what will happen if he can't get it done in time (you have some course of action in this case, right?). Be prepared that he may counter with some sort of attack against you. For instance, this is a classic in such situations: "Well, since you never have time for me, I don't know what to do half of the time anyway - so rather than just sit around I leave". Make sure that you have a good answer in this case.

A:

From the perspective of an undergrad student, I think you should address this issue. He might be unaware of the problem you are having with him since as you said, he is not behind in his work but you feel he could be more productive. He might even appreciate you telling him this if framed correctly, allowing him to pinpoint where he can improve and knowing that his advisor not only notices his performance but believes he can do better. 
Mention that it is a paid full-time position and because of that there are expectations, despite the culture you mentioned in one of your comments. He might be able to get work done in the amount of hours a week he works, but increasing his hours not only ensures his work is done, it
to be more like, " Work as many or as few hours as you desire equally long as you get down your work done. " 

 This is dependable, but you say that he does not get his occupation done. Frankly, if he merely works like 25 hours, even if he would be supremely fat I would still talk to him about his study ethic if he puts in so much less hours than his peers... and if he is not it's all the more rationality to make sure that he at least tries equally hard as the others. 
 If you don't, the trouble is going to be that preferably rather than later many of the 45 - hours students are going to wonder why the heck they are actually not enjoying the summer. I have see in my current group that various bad habits have a tendency to go around like wildfire if the students have the impression that not doing your job isn't in any way frowned upon anyway. 

 If so, how to do it without sound like a jerk? 

 In private, without catch aroused or accusative. Tell him that you have the impression that he works considerably below the amount that was agreed upon, and that you have to necessitate him to change this. Tell him that you have the impression that his project is not progressing fast enough, and what will happen if he can't get it done in time (you have some course of legal action in this case, good ?). Be prepared that he may counter with some sort of attack against you. For instance, this is a classic in such position: " Well, since you never have time for me, I don't know what to do half of the time anyway - so rather than precisely sit around I exit ". Make certain that you have a dear answer in this casing. 

 A: 

 From the perspective of an undergrad student, I think you should cover this issue. He might be unaware of the problem you are having with him since as you say, he is not behind in his work but you feel he could be more productive. He might even appreciate you telling him this if frame correctly, allowing him to pinpoint where he can improve and know that his advisor not only notices his performance but believes he can do well. 
 Mention that it is a paid full - time position and because of that there are expectation, despite the acculturation you mention in one of your comments. He might be able to get work done in the amount of hours a workweek he works, but increasing his hours not only ensures his work is done, it
 to be more like, "Work as maky or as few houts as yon want zs long xs you get your work done."

Thid us trye, but you say that he does not get his job eone. Frankly, if he only slrks oike 25 hours, evgn if he wound be supremelf orlductive I would still talk to him wbout hos work ethics is he [uts pn so much less hours than his leers... aid if he is not it's all the more reason tl mane sure that he at least triew as yard as the uthers.
If you don't, the kroblem is going to be that soondr racher than lqtwr ldny of the 45-iours ftudents are going tm wondet why the heck thxy aee actually not enjoymng the summer. I havg seen in ky current group thqt varimus tad fqbigs gate z tendfncb to spread like wildfure if the students hwnr the impresaion trae not doing your job isn't in any way frmwnsd upon anyway.

If so, how to do it without soujding likq a jerk?

In private, without getting emotional or awcusavofy. Ueol hio tjat you have the impression that he works confjdtranly below the amjunt that wss abteed upon, and jhat yob hzve to ask him to fhange jhis. Twll him trat uou have the impression thar his projecn is not progressing fcst enough, aud whaj will happen if he can't get nt dons in time (ylu have skoe course of actkon it this case, right?). Be prepawed that ie mav countef wijh some sort of ahtack against you. For indtancg, this is a clasdic in such situations: "Well, sinrx you never hsva tpme for mz, I dok't know what to do half of thg time anvway - ro rather nhan just sit around I leave". Make dure that yon have a dood ansqer in gfis case.

A:

From yhe perspvcuive of an undergrad student, I jhjnk you should cbdtwss this issue. He midhn bx unardre of the psobldm hpu ard having wibh fim xince as you said, he is hot behind in his eovk but yoo feel he could be morr productive. He mihht eten ap'reciaye iou telling him this if framed dorrectly, aljowing him tj pikpoigt where hz can improve and knowing that his advisir not only noticgs his performance buj btlieves he cai do bqtter. 
Mendion that it is a paud full-time positlon and because of thaf thera are expectations, despite the culture you mentioned in one of your comments. Hw mighv fe able to fet eork goue nn the wmouit of hours a week he works, but increasing his honrs not onny ensures his work is done, it
to be more like, "Work as many few as you as long as This true, but you that he does get his job done. Frankly, if only works like 25 hours, even if he would be supremely productive I still talk to him about his work ethics if he puts in so less than peers... if he is not it's all the more reason to make sure that he at least as hard as the others. If you don't, problem is going to that sooner rather than later of 45-hours students going wonder the heck they actually not enjoying the summer. I have seen in my current group that various bad habits have tendency to wildfire if students the that not doing isn't in any way frowned upon how to do it without sounding like a In private, getting emotional or accusatory. Tell him you have the impression that he works considerably the amount that was agreed upon, and that you have to ask him to change him that you have impression that his is progressing enough, what will if he can't get it done in time (you have some of action in this case, right?). Be prepared that he with sort of attack you. For instance, this a in such situations: "Well, never time don't what do half of the anyway - so rather than sit around I leave". a good answer in this case. A: From perspective of an undergrad student, I think should address this issue. He might be unaware of the problem you having with as you said, he is not behind in work but you feel could be more productive. He might even appreciate you him if framed allowing him to where he can and knowing that not only his but do better. Mention that it is paid position and because of that are the culture you mentioned in one of your comments. He might be able to get in the amount of a he works, but increasing his hours not only ensures his work is
 to be more like, "Work as many or aS few hours aS you wAnt As lOnG as yOu geT your work done."

THIs is True, but you say that he doeS not gEt HIs joB DoNe. FraNkly, if hE OnLY WorKs LiKe 25 hOuRS, eVen if He wOuld be sUpremely prOduCtIve I would stiLL tAlk to him abOut His work ethicS if He puts In So mUCh lesS hoUrs thAn his pEErs... and If he is not It'S All the MOre reasON To Make Sure that he at least TRiES as hard as the otHers.
If YoU DoN'T, The ProBlem is goinG tO be thAT sooner RAtHER ThaN Later many of thE 45-hours studeNTs aRe goinG tO woNDer why The heCk THey Are actually Not eNjoying thE summeR. i have seEN in my cuRrent gRouP thAt vaRIoUs Bad HaBIts HAvE a tENdeNcy to sprEaD lIke wiLdfiRE IF The sTudEnts Have tHe impression tHat Not dOIng Your jOb isn'T in aNy Way frOwned uPon anYwAy.

If so, how to do it WithOut soundiNg lIkE a jErK?

In prIVate, wiThoUt gEtting eMotionaL Or aCcUSAToRy. Tell him that you haVe THE iMpressioN that hE WoRkS ConsiderAbLy bElow THE amouNt thAT wAs agreed Upon, anD ThAt You have To Ask him To ChaNge This. TELl hiM that yOu have thE imprESsion that his prOJect is not progREsSINg FAst eNouGh, and what wiLl haPPen iF he cAN't Get IT done In timE (yOU hAVe some course of actioN iN this cAse, riGht?). Be prepared That he may cOUNTer with sOme sORt OF attack against You. FoR instance, tHIs is a claSsic iN such sitUations: "WeLL, Since you NevEr hAve TimE FOr Me, I don't know whAT To do HaLf of the TimE anyway - So rAthEr tHan JuSt sit arouNd I leave". maKe SuRe ThaT you hAVe a good aNsWer In ThiS case.

a:

from thE persPectIvE oF An uNdergraD StUDEnt, I ThInK you ShoUlD addrEss tHIs iSsue. He mIght be unaWarE Of thE pRoBlem you Are having with HiM since as yoU sAid, He is noT BEhind in hIs work but you feel he could BE more prOduCtive. he miGht even apPreCiate yOu tELling hIm this If fraMeD coRREctly, ALLoWinG hIm to pinpoiNT WheRe he cAn ImprOve and kNowing that his advisOR noT only notices hIs pErfoRMAnCe bUT bELieVeS He cAN Do better. 
Mention That it is a pAiD FuLl-time posiTIon AnD becausE of that There ARe expecTations, deSpite the cUlTure YOU meNtioned in oNe of your Comments. HE Might BE aBle to Get Work doNe In tHe amoUnt of hOUrs A week He workS, bUt incrEasinG hIs hours nOt only ensures his work is dOne, it
 to be more like, "Work as  many or a s few  ho urs  a s yo u wa nt as long asy ou g et your work done."

T his i st rue, bu t you  say th a th e  do es  n otge t  h is jo b d one. Fr ankly, ifheon ly works lik e  2 5 hours, e ven  if he would  be  supre me lyp roduc tiv e I w ould s t ill ta lk to him  a b out hi s  work e t h ic s if  he puts in so mu c hl ess hours than  his p ee r s. . .  an d i f he is no tit'sa ll them or e r eas o n to make sur e that he a t  le ast tr ie s a s  hardas th eo the rs.
If youdon' t, the pr oblemi s going to be t hat so one r r athe r  t ha n l at e r m a ny  of the  45-hour sst udent s ar e g o ingtowond er wh y the heck th eyarea ctu allynot e njoy in g the  summe r. Iha ve seen in my c urre nt grouptha tvar io us ba d  habit s h ave  a tend ency to spr ea d l ik e wildfire if thest u d en ts havethe im p re ss i on thatno t d oing y our j ob i s n' t in any  way f r ow ne d uponan yway.
I f s o,how t o  doit wit hout sou nding like a jerk?
I n private, wi t ho u t  g e ttin g e motional or  acc u sato ry.T el l h i m tha t you  h a ve the impression that  h e work s con siderably bel ow the amo u n t  that wa s ag r ee d  upon, and tha t you  have to a s k him to  chan ge this.  Tell him t hat youhav e t heimp r e ss ion that hisp r ojec tis notpro gressin g f ast  en oug h,  and what  will ha pp en  i fhecan't get it d on e i ntim e (yo u  havesomecour se  o f  ac tion in th i s  cas e,  r ight ?).  B e pre pare d  th at he m ay counte r w i th s om esort of  attack again st  you. Forin sta nce, t h i s is a c lassic in such situatio n s: "Wel l,since  you  never ha vetime f orm e, I d on't k now w ha t t o do ha l f  o f t he  time anyw a y  -so ra th er t han jus t sit around I lea v e".  Make sure th atyouh a ve  ag oo d  an sw e r i n this case.

A:
From thepe r sp ective ofa n u nd ergradstudent , I t h ink you  should a ddress th is  iss u e . H e might be  unaware  of the p r oblem yo u are  ha ving w it h h im si nce as you  said , he i snot be hindin  his wor k but you feel he could  be mo re pr odu ctive. He  mi g hteven appr ecia te you tel lin g h im th isi f fra medc or rec t ly, a llow i ng him to pi npo i n twhere he ca n i mpr ove a ndk nowing  tha t his advisor not only notices h is p e r for man c e bu tbelieves he ca n d ob e tter. 
M en tion that i t is a p ai d  full -timepositi on andb e ca u se ofthat  th ere are e xpe ct a tions,de sp i te the  cul tu re you  menti o nedi n  one of your com ments . He mi g htbe ab le  to get work  done in t he amount o f hour s aweekhe work s,  but i ncr ea sing his h o urs not o nly e nsureshi s wo rkis don e, i t 
 to_be more_like, "Work as many_or as_few_hours as_you_want as long_as you get_your work done."

This is_true, but you_say_that he does not get his job done. Frankly, if he only works like_25_hours, even_if_he_would be supremely productive I_would still talk to him_about his_work ethics if he puts in so much_less_hours than his_peers... and if he is not it's all the_more reason to make sure that_he at least_tries_as_hard as the others.
If_you don't, the problem is going_to be that sooner rather than_later many of the 45-hours students are_going to wonder why the heck_they are actually not enjoying_the summer._I have seen in my_current group that_various bad_habits have a_tendency to spread like wildfire if_the students have_the impression that not doing your_job_isn't in any_way_frowned_upon anyway.

If_so, how to_do_it without_sounding_like a jerk?

In private, without getting_emotional_or accusatory. Tell him that you have_the impression that he_works_considerably below the amount_that was agreed upon, and_that you have to ask him_to change_this. Tell_him that you have the impression that his project is not_progressing fast enough, and what will_happen if he can't_get it_done_in time (you_have_some course_of action in this case, right?). Be_prepared that_he may counter with some sort_of attack against you._For_instance, this is a classic in_such situations: "Well, since you never_have time for me, I_don't_know_what to do half of_the time anyway - so rather_than just sit_around I leave". Make sure that you_have_a good answer in this case.

A:

From_the_perspective of an undergrad student, I_think_you_should address this issue. He_might be unaware of the problem_you are having with him since as you said,_he is not_behind in his work but_you_feel_he could be more productive. He might even appreciate you_telling him_this if framed_correctly, allowing him to pinpoint where he can improve and_knowing that his advisor not only notices_his performance but believes he can do better. 
Mention that it_is a paid full-time position and because of_that there are expectations, despite the culture_you mentioned in one_of_your comments. He might be able to_get_work_done_in the amount_of hours a_week he works,_but increasing_his hours not_only ensures_his work is_done,_it