If the biological parent will not consent to your stepparent adoption you will have to ask the court for an Order terminating their parental rights.
In most cases in order to get the Judge to terminate the parental rights of the father you will need to be able to prove a few things (ex. best interests of the child for the adoption to go forward, absent parent has no relationship with the child, absent parent has not supported the child (ex., child support, gifts etc).
This can vary depending on whether the father is a putative father or a presumed father.