It is going to have to come from in himself, you can talk (if you have that kind of relationship) and maybe show some pic's of just how good some guys look in there 50's, tell him that your concerned about him and it might help if you let him know at times that you need him around and want him around.
All you can do is plant the seed, it will be up to him to make the move and then maybe train together for a bit, you know father and son thing.

Good luck man and hope he starts a new