Ask your kid to narrate what happened. After that, you can ask him questions like why did he do it, what will he feel if it was done to him. If he said he will feel bad, you can start explaining that the other kid must be feeling bad as well with what he did. Then you can ask him, will you do it again now that he knows it makes someone feel bad? The goal is to make your son realize why it is bad, and not just agree that it is bad because you say so.
Also, from the reason why he did it, you can check if there are underlying issues that might have cause it.
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