First of all, you have a great prospect and it looks like he's lready been trained really well. Kudos!
As to training him for the NT, I agree that taking his OD up to 20 is the first step. After that, it depends on if you're looking to make a PG or a SG. For a PG, I'd say if you took his PA to 15+ and his JS to 15+, you'd probably get consideration. For a SG, you'd probably want to get PA to 11+, JS to 17+ and JR to 15+, as well as getting DR up a bit. I'd estimate those to be the point where you'd really catch the manager's eye. Going beyond those numbers would give you a serious shot.
If you want to try and on the U21 next season, I imagine they'll ask you to get your JS/JR/PA all up a few levels. I'm not sure if you'd be able to qualify before the end of the season, but already having 16 OD would be a big leg up!