I don't really think a GAC will be helpful to him.
As was already mentioned, most lines are mainstreamed and there is not a 'handicapped' line; everyone waits in the same lines. There are a few attractions that have stairs, and he could avoid those with a GAC, but there are few attractions with stairs and many of those are not recommended for someone with a heart condition. Also, even though he would avoid stairs, the distance walked in line would not be less.
Just to get an idea of the distances, to go around World Showcase in Epcot is over 1.1 miles. On our last trip, I tracked the distance we walked for about 4 hours (until my ipod ran out of battery power). We didn't go fast or go anywhere out of our way, but just walking into Epcot, going around World Showcase and then back to the front of the park to ride Ellen's Energy Adventure. It was at that point that the battery was dead - but we had gone 3.67 miles before the battery gave up!
A GAC would not do anything to shorten that distance walked because most of it was not in lines - it was getting from place to place. That is why WDW recommends a wheelchair or
ECV for people who are concerned about walking distances or getting fatigued.
As was already mentioned, there are also quite a few attractions where you will need to stand for a while - mostly in preshows. Most have no place to sit unless you bring one with.
If you did not look at the disABILITIES FAQs thread, I would suggest you look there, especially post #2 which includes information about renting wheelchairs and
ECVs and post #6 which is about GACs.