You don't need a Guest Assistance Card (GAC) if you have a wheelchair or
scooter. GACs are basically a tool for Cast Members so they can tell what assistance is needed by people with
invisible disabilities or people who have needs besides just having a wheelchair accessible access. You don't need a doctor's letter to get a GAC. With crutches, a wheelchair or
ECV, they can see that he needs to use the accessible entrances.
In most cases, wheelchair or ecv users wait in the regular lines with everyone else because many lines are wheelchair/ecv accessible. These are called Mainstream lines; all the Fastpass lines, all the regular lines at the Studio and AK are Mainstream. The newer and recently renovated attractions at MK and Epcot have mostly been updated to be Mainstream. There are some that are not possible to do this to (Spaceship Earth is an example of one), so those few attractions have a separate wheelchair entrance. There are some rides where wheelchair/ecv or special needs guest have to board at the exit; in most cases, you will wait in the regular line until just before the line enters the regular boarding area. At that point, your party (of up to 6 people) will be pulled off to the exit to board. Your actual wait is usually going to be similar to the "normal" wait. If you have a wheelchair or ecv, you don't need a GAC to use those entrances.
Use Fastpass if you want to shorten your wait in line. You can send one member of your party ahead to get Fastpasses for your whole party - they just have to bring everyone's park pass to insert into the Fastpass machine.;
You can follow the link in my signature to the disABILITIES Board and then check out the disABILITIES FAQs thread for information on renting
ECVs and wheelchairs. You would be best off to rent one from an off-site rental company for him. In that thread, there also a link to the WDW Guidebooks for Guest with Disabilities and there is a link to a past thread about boarding rides (specifically the rides with moving walkways).
The rides with moving walkways can be stopped. Usually, the CM will slow them down for you to board; they don't usually stop them entirely unless you ask for that (mostly because most people can board with them with it just being slowed). The one rids that can't be stopped is Peter Pan.
If you need the ride completely stopped, you may need to wait a bit longer; some rides can only be stopped on a certain car and they also limit the number of times they stop per hour. In most cases, a wheelchair can be driven very close to the ride car so that it isn't very many steps to the ride car.