HI. I tried to send you a PM but you are set to NOT receive them. Many times, this question has deteriorated into arguments over "why should your son get special treatment vs. my child" so I wanted to pass along my experience privately, so as to not stir up the protesters!
Our son has PDD/Autism and we learned to go first thing to the Guest Services in whatever park we go to first. In Magic Kingdom, it's Town Hall on the left side past the train station.
You must bring your son with you to get the GAC and explain the situation. The reason we get one is that our son totally freaks out by standing in a line surrounded by people. We asked for and received alternate admission procedures. In MK, most of the time it is going through a handicapped entrance or fastpass line. In other parks, there may be alternate waiting areas. At Spaceship Earth for instance, you go in the "exit" doors on either side of the ball and there is a large bench that you will be directed to sit on. Your wait may be as long as waiting in line, but you are in a comfortable environment which could avoid the type of freak outs that my son experiences.
The main thing we learned because we weren't told is that every ride has a "greeting" cast member. Show the card to that CM and they will tell you what to do next. We thought it was only good for rides with a separate handicapped entrance, but in reality, they find a way to make it work at every ride.
The Disabilities forum has lots of great suggestion. There is a page at Allears. net about specifically touring with a child with autism too. Check here at the planning section
http://allears.net/pl/planning.htm About halfway down the left side, there is a section "For Travelers With Special Challenges" and there is a whole section on Autism Spectrum Disorders. That info is VERY helpful.
You'll find that most parents see a great vacation with their child with Autism. Our son absolutely loves it - he behaves better, listens better and practices good community education skills, like ordering in a restaurant.
Good luck and have a great time.