Keep the magic.
Lose the overwhelm.

Space Mountain requires transferring from the wheelchair to the ride vehicle — the seat is a tight fit. He'd need to hold himself upright for about 3 minutes in the dark with sharp turns. Cast Members at the accessible entrance will help.

If transfer is doable, go for it — that entrance skips a chunk of the indoor queue. If not, Buzz Lightyear is right there in Tomorrowland, fully wheelchair-accessible with no transfer. The PeopleMover next door is another easy win — wheelchair stays with you the whole ride.

Also worth knowing: a Location Return Time is available at the entrance if the standard queue isn't accessible — a Cast Member gives you a return window so you can wait elsewhere and come back.