For people with concerns about their ability to stand/wait in lines, WDW recommends using a wheelchair or
ECV.
The reason for that is the time spend in line is only a very short amount of the time/distance involved in a trip to WDW.
Most of the lines are accessible to wheelchairs and
ECVs, so he would be able to stay in the ECV for the whole time up until boarding. For shows, there are wheelchair/ECV parking spots so he could remain in the ECV in the line and for the show.
He
could leave the ECV outside of the lines and walk, but you should be aware that many of the queues include a very long distance to walk from the time you enter the line until you actually board/get to the show. Here's an example of one of the long lines.
This doesn't even show the entire queue and once you are finished riding, there is an equally long walk to get out. The walk would be the same whether you are using a GAC, Fastpass or the regular (standby line).
Many attractions have similar long distances to walk, even if you go at a time when there is a very short wait.
If you go to the disABILITIES FAQs thread, post #6 has a lot of information about GACs (Guest Assistance Cards) and what sort of assistance they can provide. But, as I already mentioned, an ECV or wheelchair would be a lot more help to him than a GAC would be.