Originally Posted by ScottGem
Well it sounds like he's mostly upset at mom for not letting him drive after he spent the summer earning money so he could. You might tell him you support his mother's decision and even if he comes to live with you, he still won't be able to drive until he's at least 16.
I would sit down with him at his next visit and explain to him what will happen, the fighting, the bitterness, etc, if you pursue a change in Custody. I would talk to an attorney first to get an idea of your chances.
If he still wants to go through with it, then I'm not sure you have a choice if you want to maintain your relationship with him.