I'll share some thoughts. There is not any ride where a
scooter is allowed on the ride. There are some rides where a wheelchair is permitted, but not that many. You can do shows and be in your scooter. You will use handicap parking areas for shows.
There is a designated area where the scooter is parked for the ride once you reach the ride area. You need to walk and get on the ride from that parking area. It's close to the ride, but not on top of it. You might be able to let him get closer to board and then you can go and park the scooter and you can walk to the ride instead of him. Do the same in reverse when you get off the ride. You can fetch the scooter for him. Maybe, he'll be able to do some of this himself. You'll just have to see once you are doing it.
Don't try to get a scooter that doesn't meet his weight. In fact, you should compensate for any weight that goes in the basket like a camera or coats or whatever. You will absolutely need a super large scooter and the scooter will be heavy. Hopefully, you can use Disney transportation. In my opinion you would not want to load and unload this heavy of a scooter from a car/van unless you had a ramp/ lift.
Most scooters have a lever where you can turn the seat for easier access on and off the scooter. Scooters are treated like wheelchairs when it comes to viewing areas for parades. There are roped off areas and you should be able to spot them or ask a CM or look at a map. If you want a front row you need to get there early. In most cases I would say an hour early.
I would practice back home if the scooters at the stores can handle his weight.
Most grocery stores have scooters. I would watch the battery gage very carefully for low battery. You don't want to have a scooter you have to push; because, it ran out of juice. You may have a hard time getting Disney to help if it's not their scooter. You can conserve battery power by shutting the scooter off when watching a show.
If he can walk to the park, you might be better off renting a Disney scooter. If the battery runs down or whatever a CM will help and get another scooter for him. It may take a while to replace, but it would beat any outside vendor coming into the park and replacing it. You absolutely must charge the batteries completely every night if you rent from an outside vendor.