If someone has committed assault on your son, and the adults responsible for the children refuse to deal with it, then calling the police and lay charges against the bully is an important option.

It is important that your son know that he won't be a helpless victim, and if the school authorities (e.g., the Principal) aren't trying to solve the problem then you need to go to a higher authority (e.g., local law enforcement) to get it resolved.

A lawyer can advise you on the semantics of where the responsibility lies for assault charges -- in many places if the bully is too young then the parents may be responsible (if not the school).

Also, bullies prey on the weak. One of the most effective ways to get them to back off is to come down on them like a ton of bricks. Charging them with assault is one such way to accomplish this.