us guys seem to want it all or nothing when it comes to exercise... try to come to terms with realistic goals as it is VERY important that he move or it will get worse. Even if it is sitting with legs elevated and moving the joint to its full range of motion (mine is limited so I do the best I can). He can do no or very low impact stuff like walking, elliptical machines, bicycle, weight lifting, situps, pushups, etc.
those are things I do... on good days I may do 15 - 20 flights of stairs and then none again for a week. Take a walk during lunch time at work.