Follow the link in my signature to the disABILITIES FAQs thread.
Post 6 of that thread gives all the information you need to request a Guest Assistance Card.
You do not need a doctor's notes, but some people do feel more comfortable having one - just don't be surprised if the CM at Guest Relations does not want to see it.
Most attractions have lines that are well shaded, so he will not be waiting in the sun at most attractions anyway. You also will want to look into some other things to help avoid crowds, which may be more useful than a Guest Assistance Card. Not all attractions have a quieter place to wait and, evening they do, that area can also have quite a few people in it if a lot of people with special needs arrive at the same time as you do.
- use a touring plan website to figure out the park that should be less busy for the day.
www.touringplans.com has 30 days out available for free and longer in advance if you subscribe to their service.
www.undercovertourist.com has more months out for free.
Both websites give a recommended (green) park for the day and one to avoid (red). Going to the recommended park makes a big difference in crowd levels.
- use Fastpasses. These give a much more predictable access than using a Guest Assistance Card since the card is handled differently at each attraction, based on what is available and things like how many others are waiting and staffing.
- use touring plans to figure out which attractions to see with the shortest waits. You don't necessarily have to follow the plan exactly, but using a plan can mean the difference between waiting 1 hour in line and basically walking right on for more attractions. Some popular ones -
www.touringplans.com ,
www.easywdw.com
- there are also smart phone apps that help by listing current wait times. Two popular ones are by touringplans.com and by undercovertourist.com