Question:  Under what circumstances does a child have a "presumed" father?

Answer:   The signing of a birth certificate creates merely a presumption that a person is the father. So does marriage to a man at the time the child is born. This is a mess. Since there is a "presumed" father, meaning the mother was married to someone at the time of the birth or shortly thereafter, then under most circumstances, testing would have to occur by the fourth birthday, by the bio-dad. Texas Family Code 160.607.

The short answer is that mom needs a divorce, and to add the bio-dad in the lawsuit to establish the paternity of the child.