Signing his rights away does not relieve him of the legal obligation to pay child support. If the children's mother goes to court and can prove they are his children, the court may (and probably will)(order him to pay child support) no matter what type of an agreement he has signed. What is important here is whether or not the mother signed anything waiving claims for child support in order to get him to give up his rights. If she did sign something like that, it would be almost impossible to get him to pay child support. Messy to be sure, but a judge will straighten it all out.

