I just want to echo a few comments about what has been said here. I have two sons with autism (Ages 11 & 9 at last WDW visit.) We were very concerned about air travel, hotel stays, lines, and crowds. Here are a few tips that helped us:

1.) We got each of the boys a new toy (small, hand-held) that they received on the plane. This helped us re-direct them whenever they seemed anxious. As it turned out, they flew very well.

2.) We made up lanyards with an i.d. card on them, just in case they got separated from us in the crowds. It had their names, stated that they were autistic, and directed people to call our cell phones. (Thankfully, we never needed them, but it a great comfort to Mrs. BandMan and I to know they were there.) We practiced having the boys wear them prior to the trip.

3.) Guest Assistance Pass - the greatest thing ever invented for autistic children. This allowed us to go directly into the fastpass line or in a designated 'back entry' whenever we needed to. You just go to guest services and ask. My advice there is to both bring documentation (we had a doctor's note and their school IEP) and the child, so the CM's can see that you are on the level. Apparently, there have been some unscrupulous types who've attempted to lie about their kids.

4.) Start with something you're pretty sure they're going to enjoy and work your way up to the more potentially challenging things. Having said that, you never know what is going to be hard for your loved one. We went to Crystal Palace where my oldest loved the Pooh characters, and my youngest wanted nothing to do with them. (Never saw that coming.)

5.) Remember that autistic kids often want to repeat things they enjoy. I lost count of how many times we went on Winnie the Pooh, because my oldest was in heaven there. We just tried to break it up by promising him he could come back if he tried another ride first. But be prepared to spend lots of time doing the same thing.

6.) I have to agree with the trains by Germany. My DS's also loved the trains.

Finally, let me say how much we loved it there. The boys talked about it for months afterward - and those with autistic children know what a breakthrough that can be. We are planning on returning this August, and the whole family (I also have 2 daughters, ages 15 and 6) can hardly wait to get back.

Best of luck.