I haven't seen this listed on here as a disability, but it really is.
I just wanted to mention that just because something is not listed here as a disability doesn't mean anyone thinks it isn't one.
Rather than actual diagnosis, it's easier to talk about needs. Many people with the same diagnosis might have very different needs than another person with that diagnosis.
But, people with a different diagnosis might have very similar needs - for example, schedule, quiet places to chill out , etc. might be equally important for someone with autism or someone with BiPolar.
Besides the great things already posted on this thread, you might want to go to the disABILITIES FAQs thread and check out some of the links in post #3. The ones about autism might have some advice that would be helpful to you.
A lot of the information already posted in this thread have to do with communication and pre-planning - which are going to be very important for your situation.
Another thing that will be helpful will be figuring out where it's going to be most busy and trying to avoid those places. A service like TourGuide Mike (a Theme Parks Sponsor) can really help with figuring out what is busy when so that you can avoid those places. All parks have certain times when the park is less busy or when certain attractions at that park are less busy. For example, if you try to go on Spaceship Earth in the morning, you are looking at a wait of an hour or so in line. If you go in the late afternoon, you will probably be able to ride as fast as you can walk thru the queue. For Fantasyland rides, getting there at park opening enables you to ride most without more than a few minutes wait. If you arrive after 11am, the waits are much longer as more people show up in the park.TourGuide Mike was a VIP Tour Guide with Disney and has used his knowledge about those things on his website.
Fastpasses can also help with avoiding waits in line. The other thing about coming back to a ride with a Fastpass that is nice is it's very predictable. The Fastpass has a return time printed on it for you to come back to the attraction. When you do, the wait in line is going to be usually 10-15 minutes or less.
There is information in the disABILITIES FAQs thread about Guest Assistance Cards (GACs), but one caution is that the GAC is not always handled the exact same way, even on the same attraction. It can vary based on what is available at the attraction and the conditions on that attraction at the time you came. Some people can roll with that, for others that uncertainty in itself can cause some problems. For that reason, I think TourGuide Mike would be a better option for you.
Also, I'd suggest planning with him what attractions he wants to see. That is something you can plan ahead of time and knowing can help you avoid some possible stressors - like if he wants to see a particular attraction, it's better to know ahead of time that one is being renovated. Some of the parades and shows are only available on certain days or times; that helps so you don't plan on an attraction on a Wed that is only available that week on Fri and Sat.