So I think an important question is, if he doesnt get one. How hard would it be to get one in the spur of the moment. Lets say he gets there on Monday and wakes up tuesday and says.. oh i cant do that again. Are they sold out? Do they have them easily avail?
I think wheelchairs are generally available as a back-up option. The WDW parks also have scooters for rent, but those scooters have to stay in the parks. (Though 1 voucher is good all day, even if one hops.)
There are- or at least were- multiple private electric scooter companies in the WDW area, BUT only one company has permission to leave them with bell services. For the rest, I think the small catch is that you have to be at your hotel to receive the scooter. BUT - OP would want to confirm all of the above. Off the top of my head, I don't recall which one is now the official company.
I'm inclined to think it is LIKELY he would be able to get a scooter rental on 24 hour notice or less.
If OP is thinking about having one of the smaller devices, it might make sense to have one prior to the vacation. Though I think it is possible to rent them.
My only other caveat is that operating the scooters takes a little practice. I've seen people struggle to steer and control them many times at WDW. The last time I went to HS, the poor woman in front of us kept crashing into one of the queues. I mean she chipped a chunk of wall plaster, and knocked over multiple queue stanchions. (Those are the portable poles WDW uses to mark some queues. They are connected with a chain, or by a retractable belt.)
Another option is to use assorted transportation options. Instead of taking WDW's free transportation, pay for a taxi. If OP has a car, arrive early to the parks, so they good a better parking spot, or perhaps pay for preferred parking. I think a scooter is about $50/day. Preferred parking costs $25 extra every day. There are multiple options to make a WDW trip easier. It depends how much you want to pay.
In OP's situation, I maybe would also think carefully about ADR's. Maybe what time of day is best, or just not making any, or focusing on ADR's at his hotel. If one is staying at a monorail resort, getting to the World Showcase via WDW transportation can be a long haul. If one is staying at the Contemporary, eating at the Contemporary is easy. If you don't make any ADR's, you're free to eat at any QS at any time. (or do mobile order from the TS places that currently offer mobile order to-go.)