I suggest you dial in his eating habits as much as possible, get him all the protein and complex carbs he needs to grow. Next he should adjust his training to focus on speed, power, and strength. Seriously: power and speed training will be the key to his development. Forget about bodybuilding. Forget about 10-15 rep sets. This guy should be training with HIIT routines for speed, Olympic lifts for power; he should work on acceleration constantly on all exercises, using explosive rep reversal to build starting strength. He could make great use of bands and chains in his strength training and in his high intensity conditioning. HIIT workouts will be great for him by increasing his conditioning and speed.

As long as he is into it, I bet you have a kid with great potential on your hands. The only other aspect you should focus on, which might be the most important of all, is recovery. Be sure to be proactive about injuries by training the rotator cuff, and by not putting the spine, hips, or knees into precarious situations. Use a foam roller to help recovery, and be sure to get a massage therapist to work out the scar tissue that builds up from injuries and from regular training. Deep tissue massage such as ART is great.