How do I make my son mine legally?

Originally Posted By Andrew T.My stepson's birth certificate shows no father, only the mother. He carries her maiden name as his last name. I was with his mother when she found out that she was pregnant with him and, have been there ever since. I am the only father thet he has ever known. I want to adopt him. How can I do this without alot of money. I was always told that it costs alot to adopt someone. I recently told him what the situation is and, he wants me to adopt him as well. Thank you in advance for your help.