Honestly - just go with what Andrew said.
Make it Father/Son bonding time, and hit the gym together. My Dad got me in the gym when I was 14, and I've been working out ever since (37 now). We trained together for years, and I still hit the gym with him whenever I go home. It's great time to spend together, and it's motivational for both of you. Just don't let egos get in the way and have him trying to do stuff that's too much for him, if he's new to working out.
I encourage this approach before any drugs / supplements (apart from basic stuff). Oh, and many men have allergies to whey protein, so be aware of that before he jumps all over some. Try working through his daily diet with him to encourage better eating habits, if he's not great there.