You have gotten some terrific advise so far and wish you the best of luck. We were very concerned about how our son would react on our first trip too and were pleasantly surprised. At that time we didn't do any research, didn't have any of these tips to go by (didn't know that these boards were out there or that Disney made accommodations, lol) but we made it through and have been going every year since. I also think that if you can swing it changing over to a moderate hotel is the way to go. We stay at the Fort every trip, after one at ASM that was not very successful. We love that you can drive right up to your cabin and walk up the three steps to your room, not search the parking lot with a tired, grumpy child in the back yelling about going to bed. Also, one of the highlights of our son's trip is the Disney transportation. He loves the "purple" bus, the monorail and the boat ride from the Fort to the Magic Kingdom. You don't have to get a golf cart while there (tho it makes it a lot of fun and is another highlight of my son's looping around the campsites checking out the decorations, we always go during the Christmas season). If you request being near the end of the loop, near a bus stop, it isn't bad. This just happened our first trip and it worked out wonderfully. It is nice and quiet/peaceful at this resort which we all love!!!
We always rent the special needs stroller from Orlando Stroller Rental. It has a big "hood" that can be brought down and used as a shield I guess is the best way to describe it. It has been a really great safe spot for my son when things get overwhelming. We always bring several squeeze balls for him to use when stressed. I would also recommend going on
youtube and watching videos of the different rides so he will know what to expect. Does your son sensory seek? My son loves the roller coasters because he does sensory seek. He really wanted to go on EE and was petrified after the first time on because of the yeti. We didn't even think about that (another good reason to watch the videos with your child to see how he will react to the unexpected).
We schedule our sit down reservations in the early afternoon rather than waiting for dinner so we can all get a much needed sensory break from the crowds. I agree, breaks are key and there are first aid spots where you can stop and spend some "quiet" time if need be.
I am sure that there are other things that we do that I am forgetting right at the moment as we are busy getting ready to leave for the Fort in a few days and I am trying to get everything done, lol.

Oh, one other thing that we do is pin trading. This is a great way to work social skills into the trip. In the past we have also done a couple of the tours, Behind the Seeds and the Steam Train tour, which was nice and much quieter than the parks. My son loved them both especially since they picked him to feed the fish on the Behind the Seeds tour and he got to be the engineer on the train tour.
Any other questions, please feel free to pm me. If you want to send along a pm with your addy, I would love to send him, and your other kids, a post card from one of the characters saying that they can't wait to see them. Someone on this board did this for my son a few years ago and it made his day!!!
Ronda