In humans, the less you exercise, the more your muscles weaken-maybe this is the same in dog. I would try and play fetch or ball with him. I also chase mine all over the first floor of the house and then turn around and let them chase me. Hot Rod has loved this ever since we've had him. You could also make an obstacle course for them to run around...this will help with him to think too.
Good luck and on top of everything else, the exercise will good for you too, I know I can use every bit I get. lol