We've gone to Disney lots of times and never had any problems at all obtaining a pass.
It's called a Guest Assistance Pass. Take proof of your child's disability (letter from GP with letterheading, or DLA award or dx) with you. Go to Guest Services when you first arrive at the park and they will issue the pass. It is good for all Disney Parks for the duration of your holiday. It covers you for up to 6 people (I think, as there have only ever been 5 of us), which I thought was brill. In the words of the cast member who issued us our first pass "you came to Disney as a family group, so you enjoy Disney as a family group".
For us it was a life saver as my son has real difficulty waiting in long queues. You will still have a wait, either in the Fast Pass lane or at the "handicapped entrance "(US terminology not mine) what we call, disabled entrance, but it is much shorter. I found cast members very understanding and helpful whilst using the Pass.
A wee tip, pack some plastic sandwich bags, to put the Pass in, as it will get really grubby and a bit wet after going on all the water rides.
This site, allears.com has some good tips on taking an autistic child to Disney
http://allears.net/pl/dis_aut2.htm
Enjoy yourself, I'm sure you'll love it
