I think he can easily make your U21 with at least L5 trainer and one position training. Ask your U21 manager though if he wants a PG, a SG or a well-balanced guard. My 18-yo was a tiny bit better than yours and after two seasons of training (and few weeks of the new season), he looks like this:
Weekly salary: $ 19 719
Role: rotation player
(BuzzerBeta)
DMI: 138500
Age: 20
Height: 6'2" / 188 cm
Potential: superstar
Game Shape: strong
Jump Shot: prolific Jump Range: proficient ↑
Outside Def.: sensational Handling: prolific
Driving: sensational ↑ Passing: prolific
Inside Shot: pitiful Inside Def.: average
Rebounding: pitiful Shot Blocking: atrocious
Stamina: average Free Throw: awful
He will probably be the best PG of his generation. I don't know if your could be too since USA U21 probably has better players than Slovenia, but he will be one of the top nevertheless, given proper training. One word of advice: Don't spend too much on 1 on 1 and JS training until he is 21.