Nostriches, I would recommend a high-quality whey protein powder that is easily digestible and has a good mix of essential amino acids. Whey protein is also great for muscle recovery after workouts which will help him build more lean mass and prevent injury. Some good options include Optimum Nutrition Gold Standard 100% Whey, Dymatize ISO 100 Whey Protein Isolate or Muscletech Test-HD. You can also add in some fruits like bananas, strawberries or blueberries to make it more palatable and increase the overall nutritional content of the shake. Lastly, I would encourage him to have a balanced diet with enough carbohydrates, protein, healthy fats, vitamins and minerals to support his activity levels and growth.