This app was mentioned in 8 comments, with an average of 1.38 upvotes
I used to use Fitocracy for a while. It's basically a gamified workout tracker. I only stopped using it because I found other workout trackers that were better suited for weightlifting, but Fitocracy doesn't integrate with anything else (so I couldn't import my existing logs) so it was a pain to have to re-log my workouts in Fitocracy.
OK, for real, what got me to take it seriously was Fitocracy - it turned lifting into an IRL RPG/MMPORG for me, and then it clicked with me that I shouldn't feel bad that I can't lift as much as all the other guys in the gym the same way I don't get upset that my character isn't as high-level as the others I see in WOW. I realized everyone starts at the bottom, and it's the progression that counts. Every time I go to the gym, I'm boosting my lifting stats in each exercise I do. That's all it is.
Here's the thing though. I wanted to be more fit, and fitocracy just helped me in putting lifting in the right context. If he don't have that innate desire to get in better shape, it's pointless to try to get him into better shape. So I think were you should start is in having an earnest conversation with him about whether he's actually happy with the way his body looks/performs.
Have you tried Fitocracy? It hasn't been updated in almost a year though...
Fitocracy maybe. Haven't tried it myself, but I've heard good things.