The above advice was only if you were keeping him on your team.
If you are going to try and make the U21 team and are not concerned with salary then I would do the following:
1. Train Inside Shot to Sensational
2. Train Jump Shot to prominent
3. Train ID, IS and Rebounding up to wondrous.
The reason being that I think that to make the U21 team, especially in the US, you can't spend too much time training secondaries.
However, I don't know what the current skills are for U21 players in your countries team so if his primaries are already good enough in comparison, then it would be far better for your own team to go with training secondaries.