Selket said:
The GAC question aside, I just wanted to add that an adult will be able to ride all the "kiddie" rides with your son if they wish (like Dumbo, etc.). Your son doesn't have to ride alone unless that works best for you.

The "kiddie" rides at WDW are big enough for "kids" of all sizes.
The "stamps" are the different messages on the GAC. The GAC to start with is just a card with a blank space on the front. After you tell them what your needs are, the CM uses a stamp to put a message into that blank space that tells the CMs at the rides what sort of assistance you need.
As Kathy mentioned, sometimes there isn't a stamp for what would help you the most, so the CM can write something there with the stamp.
Specifically, (I don't think this would help anyone trying to abuse the system) would it do any good to advise if possible that CM make eye contact before addressing my son. He is autistic and that is the only way he will respond to questions or follow directions. Or is that asking too much? Also, we live near a large amusement park and when my son exits kiddie rides (not with an adult) he often gets confused as to which way to go. Sometimes we can step in to help, but sometimes not depends on the ride. The ride operators tend to get annoyed with him.
Since you will be going on the rides with your child, you can be the one to address him - and he can ignore you, instead of a stranger

. (Sorry, I couldn't resist that. My DD is not autistic, but she does get into her own world sometimes and we are not sure whether or not she is ignoring us).
While you are waiting in line, you will have a chance to see how the boarding for the ride occurs. The CMs loading the rides tend to just ask how many in your party and then they will direct you to a particular place (often marked on the floor with a number) to stand. You wait there until the next boat/car/whatever comes along. For rides like Dumbo, where there is a place to enter and then you find your own Dumbo, you can watch the loading process and kind of decide your strategy.
For things like character greetings, you can tell the character or the CM working with the character things that might be helpful in dealing with your child.