thanks for your query,
as told by you your child had fracture
for which cast was applied for 3 weeks,generally after this there in weakness in concerned limb.it is usually due to disuse atrophy
of muscles,and now as he is using pot this can also put stress
on his leg as you said he is staying on pot for 30 min,
this problem will go in some time,give him proper nutrition,give him milk.give him food rich in calcium.
in case symptoms persist take him to orthopedician.
your son will be fine