Also make note that your son has a condition that warrants this request and you are most likely to get it! Bring along a note from his doctor you can probably use that to get in the handicapped isle for the rides to i seen alot of people with casts or leg problems going in the handicapped lane where you basically get right in and all you need is a dr note I believe you take it to guest services and you get a special card.
Most attractions have what are called "Mainstream Access", which means that the line is wheelchair accessible and guests using wheelchairs wait in the same line with everyone else. AK and the Studio were built with all Mainstream Lines. Newer attractions at Epcot and MK were built with Mainstream access and older attractions had Mainstream Access added if possible.
A few attractions (mostly at MK) have handicapped entrances, but those sometimes mean a longer, not a shorter wait. For example, at Small World in April, we waited an extra 25 minutes to board compared to people who could walk on. In some attractions, people with wheelchairs can't board at the regular boarding place because it is not accessible. For those attractions, boarding for people with wheelchairs are at the exit - but the 'routing' to the exit usually happens right before the 'regular' boarding place. It pretty much evens out - about 20-25% of the rides you may get on faster, about 50-60% will be the same and the rest will be longer, sometimes much longer.
The card is called a Guest Assistance Card and it is not meant to (and usually won't) shorten the wait in line. He won't need a Guest Assistance Card since the CMs can see that he has a wheelchair. Guest Assistance Cards are mostly for invisible disabilities.
Actually, there is no front of the line card - not even for those in wheelchairs - sometimes the wait is longer.
Have a look at the DisAbilities Board as they may have some suggestions = even short term still presents a valid medical need.
Hope you have a wonderful trip!
Follow the link in my signature to the disABILITIES FAQs thread. There is information about renting wheelchairs and
ECVs (power mobility scooters). Since he is over 18, you could rent a wheelchair or an
ECV for him.
There is also informaiton in that thread about Guest Assistance Cards.
They seem to think Disney always "tries" to fill requests, and if you don't get it, it's because the place you asked for is simply unavailable or your requests were unreasonable. I find this simply isn't true- it's the luck of the draw whether anyone reads your requests and fills them. 
