Step one is getting him some wheels. You could rent him a wheelchair, but that would require some BIG work from either him (wheeling himself) and/or you (pushing him). Just think about how much distance you normally walk at the parks than about combining that with the chair and not being used to that type of exercise. Yeiks. So, an option that'll be a lot more comfortable for both; renting him an
ECV.
Look at it one step at a time. Can he walk the distances from the busstop and/or parking to the park? Can he get around the resort walking? No? Rent ofsite, definately! If the answer is yes, you could rent in the parks but they are more expensive and the availability could run out.
He might want to practice somewhat, which can be done at your local
Walmart etc. by trying out one of the ecv's they've got. No one ECV is totally the same, but the basics are. Biggest things to 'get used to'; turning radius, fact that the ECV will not stop as soon as he let's go of the throttle but will keep rolling for a few more seconds, being on the look out for people around you dashing in front.
With this, he should be fine. Disney policy for stamina problems is to advice guest (rightfully so) to rent a mobility aid. With such an aid, you are all set. Your need is obvious to the CM's, so no need to get other stuff in order like for instance a GAC. On the parkmaps they now include accessibility info. Most rides have mainsteam waitlines, meaning he can enter them like he would as if he were walking. Rides that aren't using a mainstream waitline will be indicated on the map, including info on how to enter the ride. If anything is unclear; just ask the CM working the ride and they'll help you out.
You might also want to read the sticky's on this part of the board. Sue has put a treasure of info in there, any kind of info you might be looking for. Ride accessibility, pics of that, room info, pics of that, transportation info incl. pics, it goes on and on.