I am not a parent, but I was bullied.

When it gets to the point where your son no longer feels safe not only at school, but at home and in public spaces, AND you have had vandalism occur, it's time to contact the police. No child should have to face this pain every day. Ever. And there is no excuse for the bullies behavior.

I would contact the police, also the school (if they go to the school), and explain the situation. Document everything to your memory.

When it comes to bullies, I have no pity for them. If they want to live a life of freedom, they should behave like decent human beings.