I would probably just play it by ear. Late May should not necessarily be real busy and you would be able to get a wheelchair later in the day if it is necessary. (
ECVs are in shorter supply, but wheelchairs should not be a problem). If he's able to tell you, I'd ask him how he feels about using a wheelchair. He may be OK with it, or might rather just walk and take rests.
If he has frequent seizures and/or seizures without advance warning/aura, then I'd probably use a wheelchair for safety.
Another thing to think about is getting to the parks as early as possible in the morning. You would be able to get on lots of things with little to no wait. I can't get youngest DD ready early enough in the morning to do that, but DH and I have gone on "couples only" trips to WDW in the middle of summer and found it worked very well. He has also taken some members of our party to the parks early during really busy times (Spring break/Easter) and found they could cover all the Fantasyland rides, plus Splash Mountain and BTMRR before coming back to the room to rest before lunch.
During the hottest part of the day, you can be back at your hotel or in the pool. Then come back to a park in the evening when it's cooler. That avoids most of the heat and most of the crowds.
Also, using Fastpass will help avoid long waits in line. You can send one member of your party ahead to a ride with all your park passes to get Fastpasses for everyone. You and your son can be taking a break during that time or go on some other attraction. We did this on our recent trip.
You can also look in the disABILITIES FAQs thread near the top of the board for the information about GACs (Guest Assistance Cards). The GAC is not meant to shorten or eliminate waits in line, but might provide conditions that are better for your son (although using Fastpass gives more predictability - since you know when you return with a Fastpass that you will usually be riding with a wait of 10 minutes or less).