try to make his workouts short and fun, so he doesnt quit. Make sure you teach him form, and not to care about the amount of weight, and teach him that he will get stronger/bigger if he uses good form, even if he has to lower the weight, start him out with 15 reps for a couple of months, and than you can change that based on his goals. I would not let him do any overhead pressing, because at 12 he probably has a weak core, and that can lead to very dangerous things, when I started working out I was doing military presses and I fellover (because of a weak core), I was very lucky I didnt get injured. Make sure you read the sticky- Guide to a Proper Warmup, and come up with a good warm-up for him. Do you know his goals? Is he trying to get big, strong or what? Safety First.