Is it rude? How would u react?

so my kids were fighting and my ex called (their dad) and he was talking to me about their behaviour. My step son started calling him "terry taco" into the phone and i told him to stop, it was rude. my ex is not his father... hes my husbands kid aged 9. so anyway he just kept calling him terry taco even after i told him not to. my ex took offence to it and said "why the hell is he calling me that? hes so rude!" i dont know what to do ....

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