Is this 6 times over the course of the year? or 6 times in the last few weeks? Either way it needs to stop but if it has all happened in a short time frame I see it as even more serious than if the first instance happened in August/Sept...know what I mean?
Sounds like you and your son have taken the appropriate actions (teacher, supervising adult, principal, superintendent, police). I would be furious! Keep in mind though that they really can't tell you what type of punishment they are giving the other child. They should have to tell you what they are doing to keep your son safe (can't say we have suspended little Joey but can say that they have taken appropriate action and your son will not be in contact with little Joey again).
I would get out the handbook that they probably handed out at the start of school (or get a copy) and see what the handbook says. Then you can see what other steps you are "supposed to take" and what steps they should be taking (or should have already). Then you can go to them armed with thier own policies. Without knowing their policy, I wil advise the following:
Write a letter to the principal with a cc to the district superintendent and your local board of education. Site the facts (on this date this happened, aide was no where around and person on duty just said "stop", keep being told "working on it" etc). Demand a meeting with all relavent personnell (at a minimum it should be teacher and principal but should probably also include the superintendent, the recess person and the MIA aide) within a certain time frame (request it today for no later than Monday 4/29) and hand deliver all copies. If it isn't scheduled then you go to your local news station and newspaper. After telling the story and providing a copy of the letter, they may help you get that meeting.
If you have missed any step within their policy, try to take that step now.
*UPDATED* I have since read additional replies and feel the need to add that if the attacker is special needs it may change how the school handles things but it doesn't change the fact that the victim needs to be protected. IF, as some have suggested, the victim is being the agressor or bullying a special needs child that does need to be addressed but being special needs doesn't mean it's ok to just let others be hurt. If the aide was there (or any teacher supervising) they could put a stop to any bullying (regardless of who is doing it...intentional or not). Protected class does not equal "free pass" nor should it mean you have to accept your child being put at risk.
**another thought, since he's violent with everyone, can you get with some of the other parents that have had a problem and present a united front that "something has to be done to protect our children!"