School bully issue, opinions wanted

[70rs]

Ok guys, here is the scoop. My son is almost 6 and goes to an all day kidergarten program at the elementary near us. Since shortly after the start of the school year he has been the target of another kid in his class. This kid throws toys, rocks, books, punches him in the face, kicks him and pushes him down on the playground. This kid also calls him names and says mean things to my son.
This has all been documented by me and the teacher. She (teacher) has confronted the parents a few times without any change. I have been assured by the staff at the school that my son is in no way instigating any of this himself. Yesterday he got hit in the head with a book that was thrown like a frisbee. This alone could have done serious damage to his eye (hit right above it).
I have talked to the teacher, principal and today the district office directly to the principals boss. I really feel like this kid should have been pulled out of this class a long time ago and that the principal is not doing anything to fix this problem.
I made it clear today that if they do not remove this other kid from class that I will make it a legal issue for the district on Monday.
the school has a zero tolerance policy about bullies but has not enforced it at all. I feel like they don't take my sons well being seriously. I am NOT going to wait for a more serious injury to react to this. But I feel like they are.

Now before you say my son should defend himself, he knows that responding with voilence is not the right way to handle this. He does go to martial arts class 4 days a week and could wax the floor with this kid but is mature enough to know not to. Trust me, it would take all of 25 seconds for my son to resolve this on his own. But I do not want to teach him to respond like this unless it is a real emergency and he has no other choice.

So I guess my question is besides going to the district and demanding a fix for this, what else can I do? I could go to the kids house and react poorly on his parents, but that only teaches my son that the violence is ok. If he was a lot older I would encourage him to take care of it himself, but he should not be faced with that crap at his age.
I told the district office that the next step will be a legal one. I also told them that this other kid obviously needs help of some sort. 5 year olds don't do this stuff without learning it at home first.

So thoughts on how to handle this? Has anyone else dealt with anything similar in a public school? How did you handle it?

Thanks for any input. I need this resolved NOW for my sons sake.