Get him a bike and do it together. Start off short, like 400 yards, then the next day increase it, blah blah blah. Or like others said just start taking walks together. If you think you guys are in it for the long haul, it will be better for both of you if he is in better shape. Fewer medical bills to worry about, you will feel happier, blah blah blah, you already know the benefits of exercise.