First, I think you should have him try a scooter back home. You can do free scooters at the grocery stores and such. Make sure he's comfortable turning the scooter and backing up. You can visit many zoos that offer a scooter rental and the fee is nominal and it would be good practice for Disney World.
I'm not sure about how may rides would allow a walker on it. You can probably use it to get on and off, but someone else will have to get the walker back to the scooter and get it again after the ride. I'm not sure how well the cast members will tolerate the wait for someone to do this, either. In most instances, there is a small distance from where you leave the scooter and get on the ride. Because, others are boarding the rides, too Disney doesn't want the scooters right by the rides. If you can help him get to the ride from the scooter drop off area that will probably be the best answer. Another option may be to use a cane. There are canes that fold up and you probably can take them on most rides with you. You will need to practice with a cane to make sure this would be a manageable answer. You could also provide support on the opposite side for him to keep better balance. There are some rides where you could leave the scooter outside and use a wheelchair through the ride queue. Then, you could probably get him to be on the ride and then get the wheelchair to the appropriate area yourself.
Having a scooter the whole time is probably a better idea. It's a lot to get in and out of the parks and back to the resorts with your car or resort transportation. Keep in mind if you are using your car you have to take the travel, portable scooter apart and place it into the trunk of a car.
If you get an off site scooter, you will have to be diligent about charging your battery each day; so, you don't run out of juice to run it. You need to get the scooter into your room at the Disney resorts in most instances.
With a scooter you will have access to handicap viewing spots for parades, fireworks, and shows.
If you need to use a Disney bus with the scooter it will take some time to learn to get on and off the buses. You may want to go the first time time when it's less crowded to get a feel for getting on and off of the bus before doing it at a peak time. Monorails are easier and preferable for scooter travel if you can afford to stay at those resorts.
I would not attempt to use a scooter at a table service restaurant. I would do a transfer by wheelchair or I would stick with quick service places only. It's too busy and crowded to maneuver through a table service restaurant with a scooter. I have to say after 40 trips to Disney World I have never seen a scooter at a table service restaurant inside. I can see that it might work at a few restaurants like the Crystal Palace, but overall I would say it would be horrible to do this.
Doing a scooter for the first time at Disney World has challenges. There will be crowds to deal with it that will dart right in front of the scooter and he has to be able to stop quickly. You need to be well aware of your surroundings and pay attention in all directions while driving the scooter. A few rides may be difficult. Epcot's Soaring ride needs to use an elevator for the scooter to get down to the level of the ride.
A scooter will allow you to have a much longer day at the parks and in most cases it is a blessing; so, it's worth learning how to do them and use them.