Can the biological father change his mind after signing over his parental rights?

Unless the biological father can PROVE he was coerced, he shouldn't have much chance at overturning the adoption because the form that he signed clearly stated that irrevocable once signed. Have your lawyer write to his lawyer pointing this out and informing him that you intend to defend his signature as valid and that you will be suing his client (your child's bio-dad) for your ALL legal costs relating to this matter as the form he signed was quite clear that this was irrevocable. Good luck and God bless.