I know how you feel in this situation. One of my friends almost deleted my save (she's only a year younger than me). I never let anyone besides my brother touch my Animal Crossing because of this. (My brother holds precious things on AC, he could never delete it!)
I would think that he would still have known better than to touch your stuff, maybe you should try to find a hiding place for it until he grows up and realizes that's NOT equal, and that what he was about to do was a LOT more horrible than beating him at a video game. In fact, he should have gotten revenge with a rematch, not by touching your stuff. Tell him that. Now. Before it's not nearly.