Anybody can get Fast Passes (FPs). This is not related to any kind of disability or special needs.
The GAC can offer a variety of accomodations depending on his issues but it is not an express pass. The card actually says right on it that it is not intended to shorten wait times. It can make waiting easier or more bearable by eliminator a stressor though. This is why it's worth talking to Guest Relations. Explain to the CM there what kind of issue(s) you forsee him having and the CM will offer whatever accomodations are available to enable him to be able to enjoy the attractions. Note that skipping lines or not waiting are not accomodations that you can expect.
Many of us with autistic kids have found that while we do benefit from the use of GACs that we find good touring plans to be even more helpful. A good touring plan can do a lot of things that a GAC can't. There are many attractions that we would have to skip if we were there at the wrong time because the accomodations simply can't get us away from the crowds (maybe the alternate waiting area is too crowded or the FP line is too long of a wait in such a narrow space so our kids just can't handle it) but with a good touring plan we can get to the attractions we want when there just aren't any lines there to begin with so we don't even need accomodations because there aren't any stressors around with the lack of people around. In addition, a GAC can do nothing about the areas between the attractions but with a good touring plan you can be done with areas that feel really congested (something we have to avoid) before the crowds fill it up. We can't even cut through Fantasyland at certain times of the day but using a good touring plan we can easily finish that whole area of MK long before it gets congested and that's without any accomodations for our DD's autism. Yes, a GAC can make a big difference, but a GAC alone can't do everything we need and a good touring plan can take care of a lot of the things that a GAC can do better than the GAC.
Like I said, talk to Guest Relations but also look into touring plans. Both are great tools to have.
For touring plans I'm a big fan of easywdw.com but a lot of people like Touringplans.com.