For situations like your son's, having a stroller or child size wheelchair to get around the parks between attractions is going to be much more helpful than a GAC (Guest Assistance Card). Most people don't realize how large the parks are and how much walking is involved. I've heard estimates that the average person walks between 3 and 8 miles per day at WDW. Just the distance around the lake at WS at Epcot is 1.3 miles!
So, the amount of walking in lines is just a small part of the distance your family will travel each day. And, for most attractions, even if you did not have to wait in line, you would still have to walk basically the same distance to get from the ride entrance to the boarding area.
Just to agree with and add a little to what was already written:
You can go to Guest Services in any of the parks and request a Guest Assistance Card (we call them GACs to avoid typing so much). It is not meant to (and usually won't) shorten or eliminate your wait in line. What it is meant to do is provide conditions to wait that meet your needs. In your son's case, being able to use a stroller as a wheelchair would allow him to avoid standing in lines.
In general, people who use wheelchairs or
ecvs don't need and won't be given a GAC because just having the wheelchair/
ecv alerts the CM that they need an accessible entrance. If that meets the needs, there's no need for a GAC. But some people need a quieter place to wait, a place out of the sun, a place away from other people or to be able to bring a stroller into line and have it treated the same as a wheelchair. The GAC is a tool to alert CMs of that sort of invisible need.
To request a GAC, go to Guest Services in any of the parks with the person with a disability and explain your needs. Just saying, "My child has -------" does not help much, because some people with that diagnosis will have minimal problems waiting in line and others will have lots of problems. Besides, the CMs at Guest Services are lay people, not medical people. You need to be able to explain what sorts of behaviors/problems you expect to encounter/ what the needs are. That way, the CMs can figure out what they can do to help.
The GACs have different messages stamped on them, depending on what the needs are. GACs issued in one park are good in all parks and are usually valid for your entire length of stay (You don't need to get a new one each day). GACs are usually issued for the person with a disability plus up to 5 members of their party - there may be times when you are asked to split into smaller groups because of attraction capacity in the special needs area for that ride.
A 5 year old would fit in the WDW strollers, but renting one in the parks would mean he has to walk from the parking/bus area to the rental area. This can be quite a distance, especially at the end of the day.
If you bring a stroller from home, you would also have one for the resort and the airport - even if your home airport is small, the Orlando airport is quite large.
Another possibility is to rent from one of the local Orlando companies. These medical equipment places do rent Special Needs strollers (kind of like larger versions of regular strollers) and pediatric wheelchairs:
Care Medical:
http://www.caremedicalequipment.com/
Phone (407) 856-2273 Toll Free U.S and Canada (800) 741-2282
Walker Mobility:
1-888-726-6837
www.walkermobility.com
Colonial Medical
http://www.colonialmed.com/about_cms.html
(800)747-0246
There are also places that rent children's equipment that rent special needs strollers. (I lost my links, but I think you can find information on the Family Board).
You don't want to rent an adult size wheelchair for him (which some people do). Because the seat depth is adult sized, a 5 year old in an adult wheelchair either sits way back in the wheelchair with his legs sticking out straight in front or sits forward in the wheelchair, which would allow his legs to bend at the knee, but he would have no back support and his feet would be dangling. I don't think either one of those would be comfortable positions for someone with hip problems. A pediatric wheelchair or large size stroller would be more appropriate.
If you are working with a Physical Therapist, they may have some resources in your area they can point you to - like equipment lending closets where you could borrow a stroller or pediatric wheelchair for your trip. You also should ask the PT or doctor about exercises to do/how often to get moving out of the wheelchair to avoid getting stiff muscles from sitting so much druing your trip. If your son has exercises that he needs to do while lying down (or he just needs a lie-down rest), there is a First Aid station in each park that has places he can do that. The staff are really nice and the FA stations are quiet and have private cubicles.