Thanks guys. I appreciate all of the feedback. I responded that this wasn't on the table for me, thanks, and left it. He is trying to pressure and threaten me with lawsuits, which is fine, whatever. Nothing new there.
In my state there is no age rule. They can have some say-so, but they don't decide. There is no compelling reason for him to move, and a lot of reasons why not. We just renegotiated a new custody agreement at the end of winter, it is working, no compelling reason to change it.
Yes, ex has been in my son's ear since he was little telling him how great his life would be if he lived with him. His requests started a couple of years ago when my XAH and I split up. It rears it's head now whenever DS in trouble or has to do something he doesn't want to do. Of course my ex plays into it exponentially. My son is grounded from the internet this summer because of some shady things he did online at the end of the school year. The grounding was his dad's decision, but son blames me because I found him out. He's about to finish up his half of the summer with his dad and come back to my house, thus it begins again.
He's mostly a good kid, but socially awkward and emotionally immature. He wants all the spoils without doing any of the work. This is, IMO, another fantasy of a quick fix.
I've been using my Al-Anon tools to help me cope with all the yucky feelings around this.