wow he is a great prospect. I think he can be a great C or PF.
But to start off with, I'd probably train rebounding, inside shot and inside defense.
Probably something like;
3 weeks of rebounding
3 weeks of inside shot
3 weeks of inside defense
or you could simply do;
2 weeks of rebounding
2 weeks of inside shot
2 weeks of inside defense
and repeat this cycle for the entire season. But at the beginning, you might want to focus more on rebounding and inside shot rather than inside defense, as they are lower.
Next season depending on whether you want him to be a C or PF, you can decide your training procedure. If you want to make him in a PF, you need to train JS next season until it's around...proficient (should be enough at this moment), train little bit of JR and some OD.
Talk to your U-21 NT manager and he can probably give you a precise training schedule so that thss player has more chance to make U-21 NT.