Is it harder to get a first pilot certificate in a motorglider compared to a normal powered aircraft (near St. Paul, MN.)?
It will be harder to get initial training in a motor glider near St. Paul, because motor gliders are kind of rare around here.

However, consider this:

Instruction in a un-powered glider is mind-bogglingly cheap compared to power instruction. And it has the other mentioned benefits - stick and rudder skills, low age limits, etc.

After getting a first pilot certificate in a non-motor glider (and after suitable further training), one may add Motor Glider ("self-launch") privileges with only an instructor endorsement - no Designated Pilot Examiner needed!

One may also add Light Sport - Airplane privileges with a two-instructor endorsement process - also no Designated Pilot Examiner needed! If you or your son want to fly LSAs, starting with a glider rating can be a great bargain.

And you have options! Some clubs: Minnesota Soaring Club in Stanton, Red Wing Soaring Association in Osceola (* note NOT in Red Wing, despite the name); and a commercial operation: Cross Country Soaring in Faribault. Not to mention any private instructors you may ferret out in the community.

I personally am a member of RWSA and hope to get my self-launch and LSA endorsements one of these days, but I'm having plenty of fun soaring in towed gliders in the meantime.