I am not sure you can compell her to use your son's actual last name. If you can, you would file a post decree motion in your divorce case asking for a court order requiring that she use his legal last name. Alternatively, you could just contact the school or institution and give them proof of his actual name such as his birth certificate.
I think you would need to file a motion for her to use his name first; if this hasn't been done, you should file in your divorce case.
This is not legal advice. I am not your lawyer because I answer a question intended for general information only. Don't rely on what you read here without consulting an attorney. Thank you!
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