Sign over your parental rights since you aren't apart of my child life

I am a single mother. And my son dad hasn't been apart of his life. He has seen my son only a hand full of times and doesn't bother to contact him. The only thing he does is pay child support which is barely anything.  But I'd much rather him sign over his rights.  My fiance has been helping take care of my son, he treats him like that's his biological child. I really would like him to sign over his rights but I know how stubborn he is.