Yup, seconding (or thirding, forthing or fifthing) the idea of getting an
ECV. I know it can be a scary prospect, especially when you have kids to look after too, but your trip will be so much better with a set of wheels. I've been a wheelchair user for about 5 years now, but have been using chairs at Disney for a lot longer because of my dodgy joints, and I've never found anywhere better than Disney at wheelchair access

.
As others have said, renting from off-site is the best plan, as it usually works out cheaper; he'll have it to use at the hotel, Downtown Disney, etc; and they're usually easier to drive. You can get crutch-holders added to the ECV, so he can have them with him to use for boarding rides, and to get around some of the smaller shops.
The first time you go to each park, make sure you pick up the disability guide map. On the regular maps you will find little logos showing the access for each attraction (e.g. whether he will be able to stay in the ECV, will need to transfer to a wheelchair, etc), but the disability guide maps give you extra information on where to queue with a wheelchair / ECV.
I hope this helps get you started, but do come back and ask any further questions you have.
Have a great trip!
