At this point, he could probably swim. I doubt he has the kind of injury that isn’t healed enough so swimming is out. Especially if he uses a kick board and doesn’t soak himself. His mermaid obsession should draw him to it.
Maybe a trampoline? It’s a gentle jump anyway, although landing wrong could be problematic.
Yoga is gentle and you can skip poses that are difficult. There is even chair yoga, usually old ladies but hey, gotta start somewhere.
His options may be limited right now, and if so, there isn’t a lot of physical things I can think of.
Maybe Harvard has something. They probably have a lot of smart people who hate sports. I’m sure they have rowing, and it’s hard work but you sit. Kayaking is another idea. He used to love soccer but that might be hard now.
But a hobby like taking up the guitar could interest him and keep his hands busy enough to stay out of the French fries since it seems the fatter you get the less likely they want to solve it. Losing some weight should help him look better and want to try.
WOW I never would have guessed where this family stands on the "injecting people with experimental substances" issue. Thanks for clearing that up, jazz!
