End the speculation. Age verification. I have spoken with several coaches and parents that have verified the left back status of their kids but nothing is done about it. Go to the Yale Fall Bulldog tournament and you will see it on display. Unfortunately it may take a kid getting hurt to change.
This argument has been going on for years in the Long Island and Maryland threads. Bottom line, in high school you play against older kids and in college you play against men. Might as well get used to it now. Makes your son a better player.