If you son wants this, you gather up every piece of supporting documentation and/or witness to make it happen. Please be sure your motivations are on the level.
Also, it's a common misconception that children reach some magical age where they can "choose" with whom to live. That's not true. Most state statutes have an age where the court will consider the child's wishes and (allegedly) make his decision based upon the feedback of the child to ensure the reasons are legitimate, genuine, and persuasive (and that it's not about going to the "Disneyland House").