What do you do when a child is stuck in survival mode?

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How would you reach a child to teach him that "normal" people do not act like his father did with a physical response to everything? That other kids are not like his older brothers and take everything from him? That brothers and friends are different? This child thinks that he is unimportant to everyone, no one likes him, and that he is not good at anything. He has been diagnoised with PTSD, anexiety disorder and stress disorder. He is very sweet and gets along with most adults very well, however he has no social skills for dealing with peers/friends. He was not allowed to have friends and the only time he got to interact with his peers was in school. He reacts automatically and seems to be in survival mode when other kids his age disagree with him or seem to be a threat. Most times he will run away but yesterday it got physical. At school they were playing dodge ball and he was in the center with several other kids. Apparently one of the kids bumped into him or pushed him in an effort to get out of the way of being hit. Without notice he went into survuval mode and a fight started and both boys became physical. When it was broke up, he ran off the playground almost as if he was confused and ashamed that it had happened. He is getting therapy but after years of being bullied by his brothers this seems to be the only response he knows. My question is: What would you do to help him understand that there are other responses/actions he could and should take in situations like this or other situations like this? His mother talks to me and has ask my opinion but I am at a loss as things I have suggested do not seem to have an effect on the situation. He is burning his bridges with these kids at school and soon there will be no one left for him to try to be friends with.