Don't count on shorter waits, though.
All of the Fastpass lines and many of the other lines are wheelchair/ECV accessible, so wheelchair/ECV users wait in line with everyone else in the same lines for those attractions (that is called Mainstream Access). All of the attractions at AK and MGM were built with Mainstream Access. Because MK and Epcot were built much longer ago, before wheelchair access was thought of, some of the attractions there have regular entrances that are not accessible and DO have wheelchair entrances. As they renovated attractions, they added Mainstream Access if possible and wheelchair entrances are now the exception, rather than the rule.
Many people have posted that their experience was that for around 50% of attractions, their wait was the same as it would have been had they not been using an ECV/wheelchair. For about 20-25% of attractions, it was faster, but usually not that much faster. And for about 25-30% of the attractions, the wait was longer; sometimes much longer since some attractions have limited numbers of wheelchair 'spots' and there are also fire regulations regarding how many people with special needs may be in the building at one time.
Traveling for many years with one person using a wheelchair and another using an ECV, I that has pretty much been our experience too.
The disABILITIES FAQs thread has a lot of helpful information, including links to some past threads (the one about boarding in the third post of that thread may be helpful to you).
In general, if he can get out of the ECV and walk (even as little as 10 feet), he will be able to do a lot of things. What he might have trouble with depends on things like whether he can step down into a boat for boarding and back up again to get out (Pirates). The moving walkways on most rides can be slowed down or even stopped (exceptions are Peter Pan and the moving ramp that goes up to the Tomorrowland Transit Authority).
If he has not used an ECV before, I'd suggest a bit of practice before going out into traffic. He should feel comfortable turning in both directions, backing up and parallel parking. I would also suggest Epcot as a good first park to go to; it has a lot of open space and is not usually that congested. MK is about the worst; many of the walkways are fairly narrow and even during slow times, it's pretty congested. AK is also a hard park because most of the 'lands' have traffic going in both directions thru fairly narrow areas.