I would approach the coach (or even the kid) and ask him if the kid plans to wear it appropriately during the game, and if so, then I wouldn't bother unless it isn't worn appropriately when he comes on the floor during the game. I had the same situation where girls were wearing prewrap around their neck, and told me they planned to wear them as headbands (to hold their hair in place). My partner and I agreed to let it go unless they stepped on the floor with it still around their necks.