I just found out I am father to a 16.5 yr old son. His mother and step-dad are divorcing after 13 years and he is threatening to sue me for back child support. The step dad had threatened the mother with harm, during their marriage, if she ever told anyone who the father was. Right now the child is voluntarily living w/him. After I sign an acknowledgement of paternity is he entitiled to arrears? Does he have any rights to the child at all? We live in Tx.

Did he adopt the child ?

But no, the child's mother can file for child support, normally unless they already had a court order, against you, they do't give "back" support.

Next the step dad has no legal right to the child, and you could even ask for custody ( going to be hard to get) but if the mother does not want child, and step father has no legal right to child, it leave it open