So sorry this is happening to your son.
I would consider changing schools ASAP.
Some might not agree with me now but if it was me and the school still did not sort this out and it is still happening with my son and the parents of the bully ignore the situation, I would go to the school and wait for the bully with or without the parents around and scare the crap out the bully and warn him that if he doesnt leave your boy alone, then you will give him a spanking cause it seems he needs one. Tell him if his parents dont know how to teach him manners, then you will do it. He will be upset by what you have done and he will tell his parents and I can almost guarantee that they will be at the school the next day to confront you about what you did.
It seems you have really tried all you could and if your son is being traumatised by this, something needs to be done ASAP. I am talking from own experience as my LB was also bullied at school, we only found out about it when he started becoming aggresive at home and when we spoke to the school, it came out that there are children that he doesnt get along with and they hurt him. This changed him from being a very sweet secure boys to being angry all the time and he is also only 6.
Best of luck