If you let him use weights, make sure to teach him perfect form and keep the weights light for now. Obviously negatives would probably be a bad idea at this point... but a basic routine using semi-light weights that he can do 10 reps or so with would be good for overall conditioning.
My kids all like to use my weights and they mostly do dumbbell squats or curls. I've seen younger kids doing deadlifts too. What I would recommend would probably be squats, bench, bentover rows. I don't know if I would suggest military press or any other kind of press yet... bench and rows should be plenty of shoulder work. I would also highly recommend some bodyweight exercises, like chinups and pushups, even dips if you have a dip station.
Some people will tell you that weight training will stunt your son's growth but that is a load of crap. As long as you don't let him overdo it to the point where he getting injuries, he will be fine. In fact, if anything, it should help him grow stronger bones than he would have otherwise. With basic training, you will ensure he has superb conditioning for later, when he gets older and can get serious about bodybuilding/powerlifting/etc.
Make sure to stress proper nutrition as well. Remember that doing this now will effect his habits later in life, so it's a great thing to let him do with you. Also, it is good father-son bonding as well.
