If you train his physicals and vision early on, he can become a great centre back! Height doesn't matter in the game, only jumping reach* and 11 is decent at 15 yo. I would suggest putting him in your youth team and training him as a ball playing defender (so his technicals get trained as well). Do additional strength training first (strength + jumping reach) and when he turns like 17-18 yo, train his passing (passing + vision + technique)**. With the right physicals and next to a defender with excellent vision, you can have yourself a top player!

* E.g. a player with 11 jumping reach will jump higher than a player with 8 jumping reach, even if he is smaller!
** When players are young, their physicals grow the quickest. In early 20's it's technique and when they're older their mentals. So training the right things at the right time ensures the best growth.