It happened to my son, at 12u, at the Atchison tourney when he was up 7-0. I disagreed w/ the call, my son had a choice word or 2 for the ref, he later apoligized. Its a learning experience. I would think the other kid should be looking for the defensive pin before it is called, but I am not a ref, nor am I familiar with the rule book. I would say you should just be glad your son was up 9-0 and hope it is something to laugh about years from now.