As McCall said, a note from a doctor is not required.
You must have him with you when you go to thew Guest Services at whichever park you go to first. You will have to explain not his diagnoses, but what his needs are.
Skipping lines completely is not an allowable request. Some things are using a stroller as a wheelchair, avoiding noise, need to avoid prolonged lines, etc. It might allow some use of an alternate entrance or use of fastpass line without fastpass.
The GAC will be valid for the user and remaining members of the party, but not more than six people in total.
It will be valid at all the WDW parks rthrough your date of departure.