Relinquish Guardianship?

My Fiancee took in a 12 year old 6 years ago. He is a son of an ex friend that could not properly take care of him. His biological Dad now lives literaly down the street from us. He's been married now for 4 years and has been alcohol free for 5 years. They spend alot of time together now he usually spends the entire weekend at his dad's house. In the past year we have been having ALOT of problems with him in our home. We go to therapy once a week and the therapist says he has Oppositional Defiant Disorder which is very difficult to break and deal with. His Dad would like to have custody back as he can raise him now and has been active in his life even when he handed over guardianship. I really think it is in the best interest of everyone at this point to relinquish guardianship back to his biological dad. We have gotten to a point now that not a day goes by that I am not called some sort of name by him "dumb *****, fat, stupid, etc." very frustrating at times. We also have a biological child that is 2 and an adopted 5 year old. We want to have another child but I am not willing to until the things with A (don't want to use his name) get figured out. It's really not healthy to our other children to have to go threw the daily craziness that he has been causing this past year.