Does your son have a court authorized counselor? It sounds like he doesn't? With one parent pitted against the other and the child in the middle, that's no place he wants to be. There's a section in Richard Warshak's
Divorce Poison which describes how it's not surprising that the child, even one in his teens, chooses the aggressive parent hoping that then the pressuring is less. And the mentally healthy parent gets trashed. Well, that's not a good solution, his mother will pressure and tear down any good boundaries he may want to make or establish.
Sweetheart is right, in two or three years he will be legally free to break or restore any relationships he chooses. Too, soon he will probably be old enough to drive and he can "vote with his feet" pretty much regardless what the court orders. The question is whether he would choose stability, peace, healthy relationship, etc.
My lawyer (in USA) said, "Courts love counseling." If your son doesn't have a counselor then your court may agree to order counseling for him. If so, then how to reduce the risk of the ex controlling the selection so he gets an enabling, inexperienced, gullible counselor that she could sway to her side? One way that has worked in the past with courts is to be the first to present a short list of vetted counselors who have a good reputation of unbiased reporting to the court. (My lawyer stated to me that his choice for Custody evaluator was someone he'd worked with before. His words were, "He's like god to the courts, it always follows his recommendations." And he was most excellent.)
Once your short vetted list is proposed to the court then the other parent is to choose from among them only. Courts can see this as both parents involved in the selection process.
Do you think your son has been subsumed (totally taken over) or he's just trying to survive? Factors such as that may help you to determine which paths are "less bad" for you.
Just make sure you tell him you're not abandoning him or giving up on him, whatever your legal decisions are. You can bet that she's been telling him - and will keep doing so - that you don't care about him, abandoned him by choosing to remarry, and worse.