Normally, one cannot assault someone just because they are called names. In practical terms, however, there are some words that are so inflammatory that a jury might feel that someone's violent reaction to the use of a particular word or name though not legally justified would permit the jury in their own minds return a verdict of not guilty. You should contact the police and file a criminal complaint for assault and battery against your son. He will of course have to cooperate with the police and prosecutor. There is no crime of "slander" that the girls mother could have your son charged with to my knowledge. She could certainly try to hire an attorney and sue your son for slander, however, that is going to cost her and/or any attorney she would hire money, alot of money, and there is not much prospect of winning or, from the attorney's point of view, collecting from your son.