I agree with doing a upper/lower split, 4 day's a week will suffice, I would do each muscle group once a week. It is important to note that you won't actually be training each muscle group only once, because bench presses involve the triceps and front delts, shoulder presses involve the triceps as well, and pulldowns for example will activate the biceps. Squats will work your lower back, as well as deadlifts. What I would like to suggest to you and your son is to try and incorporate as many compound exercises into your routine as possible, this will also help with reducing gym time.
Hope this helps