Changing childs name and adoption.

My son is 4 years old. And his biological father has no rights to him. He just has his last name. Cause he signed the birth certificate. His biological father cannot be found. And has not been in the picture for around 3 years. I am now married and want to change my sons name to my new married name. And have my husband adopt him. I want to know how much it will cost to just change his name and for my husband to adopt him cause we don't have a lot of money and I hired it was cheaper because his biological father would not be fighting it. I live in TN.