I would start by making sure he is medically cleared. Make sure you have a note or a PARMedX from his doctor. Has he gone through physiotherapy? If he has, then he should have some exercises from that. I would start with those. Sit/stand is a good functional exercise to improve leg strength, but you may want to include some unilateral exercises like hip abduction or leg extensions as well. Does his doctor have any recommendations for cardio? This may be difficult to accommodate with the knee replacements, but probably start with walking if he is able. I would also include some upper body strengthening exercises like tubing rows or wall pushups.