I would work on his guard skills this season. I would get his Außenvert to 8, before training 1v1 Guards and Passspiel to 8. After that, I would increase Sprungwurf to at least 6. Then I would train his Außenvert and Innenvert to 10 before the end of the season.
Don't worry about his rebounding as much because our U21 doesn't need SFs with high rebounding. Also, it will take away cap space in his potential. Do you want him to be defensive, offensive or balanced?