It depends on how big he is. Several years ago, I took my former (then) 6 yr. old stepson to Disney and he wore diapers. I was able to change him in most of the public restrooms because many of them have fairly large metal changing counters, not the traditional "fold out" style changing tables. He was smaller for his age (he only weighed about 38 lbs) but even so, there would have been ample room for a bigger child. The Magic Kingdom has fewer of these larger changing tables than the other parks, but I know that there is one in the bathroom by guest relations, one larger table in the bathroom by Casey's hotdogs, and one large table over in the "Ariel's Grotto" area bathrooms.
DHS has for sure at least one very large changing table in the bathroom by the Stunt Show. Also, I believe the companion bathroom around the corner from Star Tours has a larger changing table (not all companion bathrooms have changing tables).
Epcot has a good number of larger changing tables and most of the companion restrooms have changing tables, though some are smaller than others.
Animal Kingdom is the same way -though I know for sure there is a very large changing table in the companion restroom near "Everest".