It sounds horrible, but really, other than yelling, what can the other kids really do? It's not like they are going to start a fight on the court. I don't condone any of this for one minute, but I can see how a kid would get pretty mentally tough by fighting through it. Teach your kid to stand his ground. There are going to be people his whole life trying to intimidate him and cheat him, so he has to learn how to deal with it at some point.
It's not like tennis is the only youth sport with bullies and aggressive parents.