I can tell you about what I have done to accommodate my own needs.
There is a Diamond Auto pass that
AAA gives out. It can be purchased off of ebay but that may be against the rules. The Diamond Auto pass allows you to park in special parking next to handicapped parking that is close enough that you do not have to use the trams. Very helpful for the parking situation, especially with a stroller and any disability that the trams would worsen but walking is ok.
At Magic Kingdom, there is still a need to take a monorail or a boat. The boat takes a long time. I would recommend the monorail because it does not take as long. It might be possible to walk to the contemporary and then walk from there to the park, bypassing the need to take the monorail. I'm not entirely sure about that or how long it would take.
I've had some scrutiny over why I'm asking for a GAC and I've also had over-and-above offers of more stamps than I needed. I think it depends on who you get. At the Magic Kingdom, it's better to go to City Hall to request than to request it at Guest services outside the park. If you are given a GAC, keep it to show the next trip you go because then they will know what you are asking for and that it was granted in the past.
I would recommend going to guest services and explaining it just as you have here. Severe anxiety with crowds and rides. Then let them decide if there is anything they can do. You will not know if you don't ask. I don't think you are trying to take advantage of anything.
I definitely though recommend not using the Disney buses with his anxiety issues. The buses can be incredibly crowded, standing-room only.
For rides, consider looking up the length of rides. Splash Mountain for example is a long ride. Dumbo is a short one. Another thing too is that Disney has so much more to do even if he does none of the rides at all. Lots of shows, dining, characters, etc.
Enjoy your trip.
