My kid is being bullied and doesn’t allow me to get involved because he believes this would only make matters worse

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There is this boy from his class that always makes mean comments whenever C. (my kid) has something to say. He calls him names, sarcastically laughs whenever he makes the smallest mistake, and treats him with contempt. I wouldn't have though twice before doing something to stop all this, yet C. literally begged me not to as he believes that my involvement would only embarass him in front of the class. He said it's only jokes and that it's nothing serious, but I cought him crying in the bathroom twice this month and I think this might be the reason although he denied it. How would you recommend to act in order to set things right but not lose my boy's trust and ruin our relationship?