In the long run you will probably need to do this anyway.
The cheapest rate I think you can get for a rental for a
scooter would be $25/day. Based on a 60 day rental that would be $1500. Perhaps you could get a long term rate, but it will still be better to purchase one over renting in your case. Depending on his size and weight you can get a new portable, travel scooter for $600 on up. The price varies by whether it's heavy duty including the battery and the size of the scooter including a bigger seat on bigger ones. Keep in mind the bigger the scooter the harder it will be to dismantle it and put it in and out of the car. A small scooter will go 120 pounds and usually dismantles into 4 or 5 pieces. A big scooter can be over 200 pounds and most of those don't dismantle. A popular portable brand is the Pride to go travel scooters. If he doesn't drive a car the shuttle buses or taxis can do this, as well; you will need to give advance notice about this aspect; so, they can have a proper vehicle for it. Make sure you are secured on this if he doesn't drive.
You could go to a local mobility dealer and check it out. You can see if he could get it in and out of the car (assuming he drives) by himself. You can take a test run with it at the dealer. You could even go to a local store and test one out; I don't consider the store ones exactly the same, but it will give you an idea. Travel scooters are mostly designed so you can take the battery charger in and out without having to do the whole scooter in and out. Make sure you buy one like this. You should charged the battery everyday.
A scooter is treated as a handicap vehicle; so, you will have access to the handicap ride line queues. There are designated areas for the scooters for parades (like the wheelchairs). Airlines can't charge you for this, either.
As the other poster stated you can park the scooter as you want and walk some.
If he's alone in Disney I think it would be imperative to be on the safer side and I think a scooter would be a plus in that direction.