Agreeing with the others.
One of the reasons that WDW suggests a wheelchair or ECV for "endurance" concerns is that is the best way to deal with them. A GAC won't really help that much for someone with the problems he has.
The distances between things are much larger than the distances spent in lines; having a GAC wouldn't affect the distances between things at all, and in most cases, it doesn't affect the distance you walk in lines - what it may affect is
where you wait (some GACs allow for waiting in a quieter area). There is a GAC that allows someone to avoid stairs in attractions that have stairs in the queue, but if he is using an ECV, he will not have to worry about any stairs (and very few attractions have stairs). Most attractions don't have any place to sit while you wait (and the ones that do, having a GAC doesn't guarantee you a spot to sit - the only guarantee is if you bring your seat along with you).
My FIL avoided using an ECV for many years (didn't think his problem was severe enough to warrent one, didn't want people looking at/judging him, etc). Finally 2 years ago, on the last day of our trip, he rented one at Epcot. At the end of the day, he commented that was the first day he had spent at the parks in 15 years that he didn't end up in pain. And, he saw things on that day that he hadn't seen in years, because they were too far for him to walk. He actually said he realized he had been silly and wished he had rented one a long time ago.
One of the ways to think about an ECV that has helped some people is to look at it as a
tool to help you conserve energy. Each day, when you get up, you have a certain amount of energy in your "energy bank account." Just like a checking account, the things that you do during the day either add to or subtract from your balance. If you spend too much in one place, you won't have enough for the things you want to do later. And, if you spend way too much, you will be borrowing from tomorrow's budget and will start that day with a deficit. Using an ECV allows you to save some of the energy you would use just getting from place to place and use it later for things that are more fun.
And, finally, here is a
link to a thread from a new ECV user, who had avoided using one for a long time and was so happy after using one that he just had to share.