I’m sort of in Ryan’s camp: keep the trip, but re-think it. Personally, knowing all the stress that goes into planning a vacation, I’d rather change up the details than cancel and totally start over again.
Definitely get a scooter.
Maybe he joins you for the morning, then heads back to the hotel for a long mid-day break before meeting back up for dinner (and a few rides or whatever in the middle?)
Maybe you re-shuffle the park schedule so that you spend the morning through lunch in the parks, then back to the hotel for pool and relaxation time (where he can take meds and relax)?
If he’s not a big Disney fan, and he’s primarily going to participate in the family vacation but has no direct connection to the parks, then there are likely ways that you can include him in the communal experiences (breaks, meals, showcase experiences like fireworks or shows) and still give him plenty of time to rest/medicate while some or all of you do the rides he can’t.
Also, if he could physically do the rides themselves but he can’t wait in the lines (due to scooter mobility or pain management or whatever), then look into Disney’s disability program. It would let someone from your party “check in” at a ride, get a return time based on the current standby wait, and then come back at that time to use the FP+ queue. Basically, it avoids the need to wait in the line but doesn’t actually cut the line. That may help for the big ticket things like FoP; you could check in at the ride, tell him to meet you at a specific time, and he could show up and ride with minimal fuss.
Best of luck!