Ho, leaguemate! Nice player pickup, for sure.
I broadly agree with what Big Dogs said. To expand a bit on what he said, your best shot of getting him onto the U21 team is to simply train only primaries, all the way from now, through his 21 year. If you look at the big men on this season's U21 team, they're all listed as centers. So, secondaries are nice, but in the arms race of U21 land, primaries rule.
Now your guy has a bit of an edge, already starting with 7 JS/JR, and 5 OD. Still, you should realize that if you pump his primaries nonstop, he'll be very expensive by the start of his 21 year, and huge when he's 22. So in the long term, that may not be ideal for your own club. Of course, you could then fill out his outside skills to make him into a pretty killer PF.
I'd probably do a lot of one positions IS, ID training, and then work in RB and 1v1 forwards in short weeks (would be nice to get that driving up a bit, even though I'm not a huge proponent of DR on bigs). Ideally a couple weeks of passing training would be good too. That will take away a bit from his primary skills, but keep his salary a bit more manageable, and he'll be a more effective player overall.