I don't think there is a good solution. For a primary school boy, you can be bullied or a bully yourself or both. Intentional violence is a meme. It spreads like a virus among the school children. It only takes one to have had it and then it spreads. I can offer you sympathy in this, but I would much rather you prove me wrong. Please tell me I am wrong. ;)

On the other hand, there is the expectation for intentional violence that now your child may feel and incidental pushing may seem like attacks when they are not. Deescalation without capitulation is important. Getting him into groups outside of school is great idea.

It seems to calm down in adolescence for the most part. Maybe that is an early parental instinct kicking in or something.