Parental Responsibility, is it possible to change this?

I am divorced and my ex husband has not seen his son for over 7 years, I am now going to remarry and would like to know if it is possible to change the parental responsibility from the birth dad. When I remarry my son will be the only one with a different surname and we would like him to feel included totally in the family. He views his step-dad as his dad as he was only 2 when he became part of our lives. I don't know where his birth dad is and am not sure how I can trace him. I don't think that he would agree to my new husband adopting him as he is very spiteful and wouldn't do anything to make us happy but I wondered if I could apply to a court to have his rights removed as in affect he has abandoned his son? UK answers only please.