MinniMama, you will probably be walking to Epcot and maybe the Studios, and if he has an ECV from an off-site provider he can just drive it there. This is what my Mom did when we stayed at the Boardwalk. And if he wants, he can even drive it through Epcot and take the monorail (very easy to load and unload and ECV) to the Magic Kingdom.
Also, he doesn't have to be the one to drive it on the bus...any member of your group can do that for him. Mom was worried, but then when she found out how easy the boats and monorail were to manage, she didn't worry any more.
Plus if you park hop, there is no guarantee that there will be another ECV at the next park, which can be a big problem.
Good luck!