At the same time, you should explain to your son that the other children are still learning how to behave and that he should tell them to stop, but if they continue throwing things or hitting people, he should play with other children. If he continues to have problems with bullies as he enters elementary school, you may want to consider enrolling him in a social skills training group. Finally, I would talk to the parents of the children who are bullying your son. Sometimes parents are unaware of their children's bullying behavior. A friendly call, done with sensitivity, can help alert them to the problem behavior.
Bullying is, unfortunately, relatively common in childhood.
However, just because it is common does not mean it is harmless. Parents and teachers need to take bullying seriously - not only for the sake of the victim, but for the sake of the bully as well.