Hey there! I've got Fibro and ME and am also a Disney nut! I've been using a wheelchair for at least part of our trips for almost 20 years (wow, that makes me feel old) and I've never found anywhere more accessible and accommodating. I think a wheelchair will be a huge help, even if he decides not to use it the whole time.
One piece of advice I can give you is, use the first aid centres. They are brilliant and will do what they can to take good care of you. If you just go in and explain the situation, they'll give your son a bed to rest in, as and when he needs. It's cool and quiet, and he can stay there for an hour or so if he needs to. I've used them a number of times when I've been flagging, and they've been a lifesaver - so much easier than trekking back to the hotel!
Other than that, the main tips I can give you are probably ones you already know. Keep him well hydrated and fed (my appetite often goes in the heat, until I crash from hunger), and take plenty of breaks in the aircon. I find sports drinks very useful (not the kind with caffeine in) as the electrolytes and sugars keep me ticking. Use the wheelchair in lines and parade viewing areas - it's not cheating, it's just saving energy. And most of all - have fun!