Hi. I'm the mom of a 7yo DS with Asperger's Syndrome. We travel to WDW every 8 months or so and find that it's best to stay on site with a car. The first few trips (our 5th in just over 2 years is 9/28 through 10/4) we used Disney transport nearly exclusively. We've found that having our car at the ready helps out when we have meltdown issues.
We try to follow my son's lead in most cases and look for the signs that a storm is brewing. He's afraid to try new things, but once he does, we can't get him to stop doing the new thing!
A pool is a must for us. We get a lot of mileage out of a couple of pool mornings, or if we take a dip before bed.
We keep a refrigerated cooler in our room with favorite foods in it. We have other issues (severe allergies to food dyes) that keep us on the watch for what goes in my son's mouth. We stock the cooler with his favorite yogurts, fruit, cheese, wine (for mom and dad) and some sandwich meats. It helps out when he's having a 'I don't want to try anything new" day. He also barely sleeps, so if he wakes at 6 am and we're sleeping, he can get a yogurt out and not pester us to take him for breakfast!
We also use the park strollers, although you may want to bring your own. The helpful thing about the park strollers is that they are covered and 'enclose' your child on three sides...it cuts out that excess stimuli. My son isn't a big hat fan, but at WDW he wears a baseball cap almost constantly. Look at ways you can help him cut out all the stimuli that bother him...each of our kids is different. My son has issues with toilet flushing and hand clapping, but most other things don't seem to bother him.
There's a link on here somewhere about the Guest Assistance Card, GAC for short. It will help on those occasions that you encounter lines and other situations that your son may react poorly in.
Get a Birnbaum guide and start researching. Involve your son. Including him in the planning will help things go smoothly once you arrive. We have the park planning videos (and DVD) and those bring the park to your child and give him an idea of what's in store.
If you have meltdowns occur, it's helpful to find a nice out of the way nook (they are plentiful--trust me) to get your child calmed down. We have found quite a few areas to do this over time.
Feel free to ask away here on the boards. We're here to help!
Suzanne