Besides, I am sure he is playing as CB for the youth team and will do so when he plays on loan somewhere. Why not try him as LB, he surprsed EVERYONE how well he did as a RB.
Sorensen is pretty solid defensive wise for his age, now is the perfect time to acclimate him to another position on the backline. As far as the athleticism, of course you can't teach something the body can't handle.