I have been at Guest Relations waiting in line twice when I saw someone who was denied a GAC.
The first case, they had a letter from the MD which was just a blanket letter saying the patient has 'xxxxx and can't wait in line.' when asked for more details about what the needs were, the person could not provide any further information. In the second case, they had no letter, but also were not able to explain what the needs were or really answer any questions from the CM. There is not a requirement for WDW to provide a GAC just because someone asks for one - needs the person can't or won't explain don't have to be provided.
If he can explain his needs for a GAC and if WDW has a way to provide those needs that are reasonable, he should not have a problem getting a GAC.
For many reasons listed in previous posts, I would recommend checking into things like touring plans and using Fastpass. Those will actually be helpful in more wats and more situations than a GAC would be.[/Q
Until our last trip I wasn't aware that the GAC even existed. Until then we did use FP and Touringplans. It did not work well for him. During our trip in Feb 2010, which was during Pres Week my husband gave up visiting attractions after 2 days and stayed at the resort while my son and I visited the parks. The GAC last visit was a blessing. Not only did it allow him to enjoy the attractions but helped him not to be so worried about not making it to the restroom before his ostomy started leaking.