The ship may have sailed for U21(but maybe not) but he can develop into a NT guy becuase of his potential. Because of his height, it is best that he becomes a PF/C, not a SG(Though SF isn't out of the question).The good thing for you, if you want to make him a big, is that the training plan is pretty simple. He has good secondaries, so just train Inside Scoring(single position for Centers), Inside Defense(single position for Centers) and Rebounding, for the next several seasons and see where he ends up. (Don't worry too much about Shot Blocking, many NT guys don't care too much about it). Though you do need to find some time fore Stamina training. Good prospect you have there. Good luck training him.
Check the Suggestions they are important