You definitely wouldn't be able to carry it on the buses. If you rent a car, you'd be able to transport it that way, for sure.
If you can bring yourself to rent offsite, while there will be hassle with the buses (although, I didn't find the buses a hassle as a wheelchair user, but that was definitely off season), there are some definite perks.
ECVs from the offsite rentals are far easier to handle than the ones WDW rents. You'll definitely have the
ECV if he needs it, which won't necessarily happen if you rent in the parks because they do run out often. You'll have it if he needs it at the resort; the resorts can be big and if his arthritis flares, that could be an issue. It's also not necessarily a close walk to get to the
ECV rental once you do get to the parks. Also, it's substantially cheaper. An ECV rented from WDW is $50/day plus a $20 deposit that you get back at the end of the day. That means 7 park days is $350. You can rent from offsite for that long at a price under $200.
Especially if you rent onsite (but renting offsite as well), I'd really suggest you remind him to use it before he needs it. Yes, he might be able to walk more, but using the ECV will give him more energy and it's always better to try to not push to a level of pain/increased pain. It's much easier to keep a low pain level steady than to bring down a high pain level and pain does also drain energy. I know you didn't ask about this, but it really does make a difference and at WDW it's easy to walk more than you realize. I'm not saying he shouldn't walk, just that he should pick his times and pay attention to things like distance through a line (there are some that are about 1/4 mile from the entrance to the boarding point and then the same back out). Many people find that using an ECV between lands and walking in them (possibly with the exceptions of the rides with long distances in their queue), works well.