Can my partner adopt my son from a previous relationship.?

My sons 7 i never knew his biological dad it was one of those things that just happened.Therefore he is not named on the birth certificate and has no legal rights as i said never knew him.Me and my partner have been together for 4 years were engaged to be married we also have a 1 year old daughter.My son calls my partner dad and doesnt know no different has he the only dad hes ever known.We changed my sons name by dee poll to my partners surname last year.We just want to know if it possible for him to adopt him without us being able to contact the biological dad as we dont know who he is.