Thanks for the advice!
here is what the U21 coach replied me:
Depends on his other skills at the time but for your guy, at least 11/7 JS/JR. That sounds like a realistic goal but its not a guarantee he'll get on the team. He'd need exceptional OD (Marvelous would be a good target) or exceptional Passing (13/14 range and high OD) or some unique inside skills.
Other guys are usually around 14/9 give or take in either.
Well...he will definitely be an important part of my team in the future...that's all i need to know...rather he makes the U21 or not...knowing someone like Ken Wilson was only a backup point guard for the U21 team when he was 21 made my goal faded away. haha