What is the probability of having a son with blood type O?
7/16 for any child to be blood group O, half of that (21.875%) for a son with blood type O.
P  x  M
HA    HB
HO    HO
hA    hB
hO    hO
HA = 0/4 blood type O
HO = 2/4 blood type O (HO-HO, HO-hO)
hA = 2/4 blood type O (hA-hB, hA-hO)
hO = 3/4 blood type O  (hO-HO, hO-hB,hO-ho)
= 7/16 chance of type O
son = 1/2
Chance of blood group O = 7/16 * 1/2 = 7/32 = ~0.22
Edit: as it's not clear from the question, anyone who has genotype hh is not able to express either A or B antigens, so is always blood group O.