Rest is very important for muscle recovery and as your son mentioned when your resting, you are repairing your muscles which will make you stronger. This is one of the reasons most weight lifters have moved instead of a full body workout to a split routine of things like, back and biceps on Monday. chest and triceps on Wednesday, and legs and core (abs) on Friday. Tuesday and Thursday would be strictly cardio, but how you isolate the body parts you are actually giving them time to rest for several days while working on other body parts. I would recommend looking into a bodybuilding website, alot of information available and obviously they know what they are doing to build muscle.