personally I think your 18 yo hofer is worth much much much more than your 21 yo trainee, so you really should direct all your training plan towards your 18yo to maximize his worth.
I'll start with 2 seasons of 1on1 (either guard or forward depending on whether you want your player to play more outside oriented or inside oriented). At the beginning of the third season you can then decide whether you want to 1on1 for one more season or switch to OD/PS and then other skills.