I started training my grandson at age 12. I agree with one comment....stay off the weights. Sit ups, pushups, skip rope, medicine ball, and wind sprints, were the exercises of choice before moving into the boxing drills and hitting bags. I admit that we pushed him to hard on the wind sprints at first and he would began chocking and could not catch his breath. Athletic ashma. He had to get an inhailer because his lungs were not developed enough for the heavy wind sprint drills. In six months time, he could run sprints and distance runs with no problems and no inhailer needed. Make sure his diet is balanced. At his age, he could burn it off quickly, so I was never concerned about to much carbs, protein and sweets ( bit I did discourage soft drinks and cool aide). Add some fun to his training such as dodge ball and wrestling with others on the mats. If you ever do start him out with weight training, be danmned careful about overloading him to quickly. That age requires attention so you do not burn him out or cause injury...keep reminding yourself...he is young..there is plenty of time to develop him.