U need to stress:
1) this is bully behavior, which is not acceptable in school. As parents, u will not tolerate the situation.
2) the school is responsible of every child 's safety. They need to have a plan, instead of putting innocent children in danger.
3) ask principal to provide a plan of preventing same thing from happening again. Can your son change class?
4) coach your son to stay away from the child and report every incident to teacher and parents.