Just go to Guest Services in any of the parks and talk to the CM there about your sons problems and needs. The diagnosis is not really that important because people with the same diagnosis can have very different needs. The GAC is not given based on diagnosis at all (for example, there is not a "list" of appropriate diagnosis for a GAC); the GAC is given based on needs.
Afew other things
- think of the GAC like an insurance card, not necessarily needed or used all the time, but there for when you need it.
- even on the same attraction, the GAC is not handled the same each time. Exactly what happens depends on how busy it is, how many other people with special needs are there at the time and staffing.
- if you/your child needs a lot of consistency in what will happen, getting Fastpasses will give the most consistency. You don't have to be present to get a fastpass, you can send one member of your party ahead with all the park passes to get fastpasses. When you report back to the ride at your fastpass return time, your wait will be 15 minutes or less.