I read a lot about people who did hydrotherapy with birds once their bird had stabilized. But I didn't do anything specific to rehab my girl. Her demeanor is such that if you present her with a challenge, she'll figure it out. So I've just kind of adapted her environment as she healed -- first everything on the bottom, then platforms/perches very low, then a little higher, etc and now she's in a setup that's barely adapted at all besides having the perches closer together because she's not good at cage bars. She basically rehab'ed herself. I'd say for now just concentrate on supportive care -- pad the cage bottom with towels or fleece, move everything he needs down, have a source of supplemental heat if the vet thinks he needs it. Make sure he's not at risk of falling into/drowning in his water if he can't hold his head up. But basically let him call the shots and the two of you will figure it out as he recovers and adapts.