What I tell managers is that implementing a training plan for young players depends on the need of your team. This is because not only does it set you up for a solid player in the future, it also gives incentives to managers to keep training their players which would help the U21 team and maybe even the NT.
The potential and cap limit of a player should be considered as well.
However, if you're unsure of what you need out of the player that best fits your team, I generally go based on the minimum standards I set for each position based their starting skills.
In this case, this player would be best suited as a well-rounded guard.
1. I would start off with training JR (which should give some elastic effect with JS) to around 6. This is because fully neglecting JR and training all out JS will make it harder for JR pops in the future.
2. Then I'd focus on PAS and OD for half a season* each which should get to around 10 or 11.
3. Then I'd focus on one on one guards to get pops in DRI, HAN and the elastic effect on JS.
*The period of time provided could also depend on the level of your trainer and if you have any youth trainers which assists in extra pops for 18 and 19 year olds.
Considering you want him in the U21 team in 2 seasons, the above training plan should suffice but as a general rule of thumb, I like PGs having 12 OD and PAS, 10 DRI, HAN and JS and 7/8 JR.