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Work with your son by giving him suitable lines and phrases that assist him to speak up for himself. Role-play situations with him and train him to speak up. He will find adulthood a lot more satisfying if he can learn to address bullying now, rather than have to address it when he is a full-grown man.

I’d resist trying to make the school the guardians of his appropriately fragile ego since it appears that your son is experiencing life within the bounds of what is “normal” or common behavior among children of this age. I am sorry life can be so tough even when a child is 9-years-old, but the sooner the boy gets to handle it for himself, the better he will feel about himself and the less likely it will be that other children will target him.