with his size, JS and OD, there's little choice but making him a C. Be aware that keeping him and making the U21 might be mutually exclusive.
That said, some serious Passing and JS training now (up to about inept-mediocre each) will cost you next to nothing more next season while improving him quite a bit. Then next season, start pounding his inside skills as long as you can afford to do so. You will want him to have prominent IS, ID and Rebounding when he turns 20, and about tremendous in all these skills when he turns 21. Hopefully, you have promoted once or twice until then, because he'll cost you about 50k by then. You could then go on training him inside, as his potential allows him to reach a salary well above 100k, or pump his JS and Driving a little further so he could also play PF on offense (though he'll be restricted to playing C defensively)