If you haven't already, check out the disABILITIES FAQs thread located near the top of this board. That has information about GACs (Guest Assistance Cards). One of the most useful things for you might be to be able to use his stroller as a wheelchair (keep it in line with you and be treated the same way wheelchairs are treated). Having the stroller will give him a little "safe haven" that might help prevent some of his panic.
Also, check out the links in the disABILITIES FAQs thread to the WDW Guidebooks for Guests with Disabilities at the Disney website. Get familiar with the attractions so you know a little what to expect. Look things up on the
height calculator so you know ahead of time which things he is too small for.
If you know specific things that bother him (like fire, explosions, loud noises, etc), you can ask specific questions on this board. There is also a guidebook you can find in bookstores and libraries called something like "WDW for Kids, by Kids". Obviously, he's too young to get much out of the book, but the ratings by kids may help you decide which attractions he might like and whether they have any scary components.
There is also a very good DVD of
Disney Sing Along Songs called
Disneyland Fun - so it is DL, but the attractions are similar enough that most people (even adults) don't notice that much. The DVD does show some of the scary rides (like Haunted Mansion) and models the kids getting onto the ride. It also models children interacting with the characters (which may be helpful because many kids are intimidated by the huge size of the characters).
If he's startting to use any PECs (Picture Communication), you can find many pictures on the internet.
Hope you have a great time (and keep coming back here with any questions).