If he has never used an
ECV before, have him practice at places like Target,
Walmart or grocery stores that have them for customers who have mobility problems. Those
ECVs are much bigger and harder to manouver than the ones you can rent, plus they have a big cart stuck on the front. If he is able to get to the point where he feels fairly comfortable on them, using one of the rental ones will be
much easier.
Then, when he does get to the resort, have him practice driving a bit in a quiet place. He will need to be able to feel fairly comfortable going straight, turning in both directions and backing into a spot from both directions. He will need to be able to do this both for the buses and to pull into parking spaces for shows. Most people pick these skills up fairly quickly, especially if they have driven a car before.
When he's ready to venture out to the parks, Epcot is the easiest to start with. It is least congested and has the widest paths. AK and MK are hardest because they have narrow paths and MK gets very busy and congested.
If you stay at one of the Epcot resorts, there are walking paths and boats to get to both MGM Studio and Epcot. The boats are easy to get on with a wheelchair or ECV. The monorails are also easy with an ECV or wheelchair.
If you stay at one of the MK monorail resorts, you can use the monorails (again, easy to use with a wheelchair or ECV) to get to both MK and Epcot.
There is information about using wheelchairs/ECVs with the buses on the disABILITIES FAQs thread. You can get there by following the link in my signature to get to the disABILITIES Board; the FAQs thread is near the top of the board.