I live in Colorado and have a 13 yr old son. His biological father has never seen him and is not on the birth certificate although he knows he is the father of my son. I am now married and my husband would like to legally adopt my son. I have tried searching online for his biological father but with no luck. From my understanding I need to find his father in order to officially establish paternity so that I can then try to have his parental rights revoked. What should I do? It has been 13 years... I have no idea how to find him.

You go to Court, prove what you have done to locate him (usually using a private investigator) and ask that he be served by publication. The Court - if they award that relief - will advise you of the proper wording, appropriate newspapers, all you need to know.

You don't actually "have" to find him but you will have to make an effort that satisfies the courts needs. It must be fully recorded and tracked. And it may require notification through publication in his last known area. Were you married to him?