As inky dinky doo mentioned, there's no need to leave the DIS site for this info.
Here's a
link to the FAQs for the disABILITIES Board on the DIS Boards. One of entries has everything you would need to know about renting from either the parks or one of the off-site rental places. Another entry is some helpful links for people who are traveling with wheelchairs /
ecvs or disabilities.
There's usually a link in my signature for the disABILITIES Board, but signatures are not working right now, so you can follow the link that inky dinky doo provided if you have more questions.
For transportation, have him practice (and become a bit comfortable with) backing up before venturing out the first time. Almost 100% of the buses are equipped with either lifts or ramps to load wheelchairs/ecvs. The older buses have lifts and he will need to back onto the lift. The newer buses have ramps. Most people report the first few times getting on the bus are a little intimidating, but after that it's easy.
For transportation from MGM Studio and Epcot, you can either take the boat or there is a walking path that the
ecv can be driven on. It's about a 5-10 min walk to the Studio and 10-15 to Epcot. If you want to take the boat, they are accessible, drive right on with the ecv.