Quote Quoting Taxing Matters
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One possible exception to that, however, is if there is no legally determined father. The OP says that he and the mother were never married. As a result, one of two things generally had to happen for him to be determined the legal father of the child: (1) he and the mother executed a voluntary affidavit of paternity or (2) a petition is filed in court by the mother or the putative father to determine paternity and the court rules that the putative father is indeed the father of the child. He does not say that he did either one, so it is possible that right now he is not the legal father of the child. Until he is the legal father, he has no rights under the law with regard to that child but also does not have an obligation of support either.
In addition, there is also the possibility of being served by publication if at some point in the scenario the OP was no where to be found by the mother.

So, there are ways that mom could have obtained a legal name change for the child, even without dad's consent. Odds are that mom did not actually do so and the child is simply using a different last name, but it cannot be ruled out, at least with the information we have so far.