can you have an adoption of an adult child annuled?

What is the name of your state? Louisiana. I need to know if it is possible to have an adoption annuled of an adult child. He is 19, related, a cousin, but my parents adopted him when he was a baby. He is looking into having his name changed, to his biological father's last name. He doesnt' even refer to us as his family anymore, and calls my parents by their first names when he even bothers to speak to them. He has moved in with his biological mother. It's a very long story, but my father has cancer, and my mother is looking to protect what is theirs, in case one of them passes away. Here in Louisiana we have the forced heirship law. If one of them were to pass away, he could take his part of the estate whenever he wanted, even if one parent is still alive. He has been in trouble before, and I would not put it past him to do this if he gets the chance. Also, if he does legally change his last name, does this still entitle him to be an heir? Any help or advice would be appreciated!
Thank you.