he's got good stats but i'm afraid to say that he's too small to do any major training
I disagree strongly. The height will slow down training, sure, but right now on the transfer list there are 8 PF/C 6'8 or shorter with salary over $100K. With HOF potential, this is definitely a prospect worth training.
Back to the OP's questions. I'm a guard trainer, so I can't advise you how to train this player in detail. Strong SB is likely enough, though, and you should concentrate on IS, ID, and RB. You'll probably get another pop or two through ID training, anyway. Train single-position if you can to get maximum benefit. If you decide to train single-position, I find the best way to optimize his minutes is a full 48 minutes at the position you train (C) in one game and reserve (PF) in another. This will keep you comfortably in the 60-70 minute range to get the best game shape for your player.
Finally, reach out to Jelme (U21 manager) if you haven't already to let him know about your player.