My son also faced this in HS through to the professional level (at least in the minors). There is no one right way to handle it (IMHO) and more often than not requires our boys to be the bigger men even if they are decades younger than the coach. However, at the high school level, if it is persistent, if the behavior is mirrored or adopted by a majority of the team, and if it negatively impacts opportunities for your son, action may be warranted. Decide with your son what that looks like. More than anything, we have to prepare them for the world as it is and one of things they will have to deal with is ignorance. I still firmly believe there is far more good in the world than bad.