I would seriously consider renting a wheelchair for your son. If he is not used to using crutches, they can be very tiring and painful by the end of a long day, even moreso at Disney with all the distances and standing.
If your son is young, then consider renting a pediatric wheelchair which is reported to be much more comfortable for small children and even very petite adults. Pediatric chairs are available from offsite vendors.
If you son would like to stretch his good leg, he can always bring the crutches and use them in short spurts.