I would advise at that age he should focus on speed development, running economy, sprint mechanics.
I would add volume very gradually.
Being efficient means being faster at shorter distances so you're more efficient and smoother at a longer less demanding speed. For instance having 21sec 200m speed will make 45/46 feel pretty smooth and then coming thru 50 in an 800 isn't is a breeze. By then an athlete of this caliber (1:44-46) has a solid volume base behind them.
I would also work on his strength with weights, plyos, core.
Lunges, squats, box jumps, hurdle mobility, bounding drills, etc