First of all, most babies slim down once they get active. I've heard of chubby babies actually going DOWN in diaper size when they shoot up- even more common is to fit into the same size for a long, long time. Of course, this is more the case for chubby toddlers- it sounds like your son is just big all over, since he's about the size mine was at age 3 (when we were working on potty learning.) Still, he might stay in the same size diapers for the next 2 years.
If not, try doing a websearch on "adult cloth diapers" and look for "youth" sizes. You could also try sewing your own!