Prior to my wife having a son, the biological father abandoned her and did not want to have the child in his life. After her son was born she had placed only her surname as the child's last name, and did not have a father listed on the birth certificate. Several months later she obtained legal documents from the biological father giving up all is custody rights. However, she was able to obtain court ordered child support payments, and is currently still receiving them. He never met his son and has skipped on several payments. Also, my wife was never married to this person.
My wife and I have been married for 2 1/2 years now. I accept her son as my own, and not as a stepchild. He also has strong feelings for me.
My question is: Can my son have his last name changed easily to mine, or do I need to adopt him in order to do that? What would be the best route to go? What complications could I expect?
Thanks for reading my post.

