What about Soarin? He is strong enough to keep his leg straight for the 4 minutes. Will they let him?
He can keep his leg straight out at Soarin if he wants. My kids always kick their legs up and down - no reason you can't leave them up (seems like that would hurt alot though?).
I have a bad knee, so often try to place my right leg straight. The boat rides are good - assuming his family isn't squished right next to him (Pirate, Jungle, IASW, Mexico), the dark rides like Pooh and Pan are easy to go straight legged, and Movie Ride. Figment - definitely; Spaceship Earth if he rides alone (not straight leg but not too bad). Skip 7DMT - I'm 5'6" and smaller build and my knee cries in its tight seating. All the movies (2 at Epcot where you sit at France and US; Hall of Presidents at MK); Carousel of Progress; People Mover; Mermaid dark ride at MK; Enchanted Tales with Belle.
YMMV if its a very crowded day and CMs are packing people in, but i do keep my right leg relatively straight (not totally straight) on many rides and attractions. And, I'm not talking perfectly straight leg out in front of me - its tilted down to the floor of the ride vehicle, but still mostly straight. Oh - and he can meet characters - that's all standing of course. HTH.
PS I ride Buzz, TSMM, BTMRR alone - and with my right leg mostly straight - because we are a party of 3. (7DMT will make me sit with a stranger - every time - but no other ride has.) If its a very busy day not sure they will let him ride alone unless he presents as just a party of 3. If not busy, just mention his surgery and that it would be a huge help if he could ride alone.
Basically he will need to relay on the rest of his group to squash over to give him room, if its a busy park day and they are packing the rides. I definitely recommend rope drop. Lower crowds = less packing in on rides, even the boats and dark rides.
OH - and he can do DS's favorites: Tiki room and country bear Jamboree! I even take my shoes off and put my leg on the bench in Jamboree.