14 year olds can have the emotional maturity... but then it knocks them for 6. I gave a LOT of thought to moving to live with my mum (yes, it was the 'other way around'). They did talk to me about it, perhaps not in an unbiased way, although my sibling who had moved with them a year or so earlier tried to dissuade me (and when I did move in told me I had ruined everything). I had to deal with my dad telling me that he would kill himself if I left. It took a HUGE amount of courage to say I needed to go and that if he chose to do that it would be his choice (or something like that, I don't remember much from the conversation, except that my heart had been ripped out my chest and shoved back down my throat). He emotionally abused me tho, and it was my gran (maternal side), who actually raised me. I didn't want to leave her, or my DB, but I had to leave him. I'd already started harming myself, and I was sensible and EMOITIONALLY aware enough to know what to do. But boy was it SO VERY HARD. And yes it left it's scars.
Totally different know, but teenagers really are capable of making hard decisions.
BTW my dad did not kill himself, but after that conversation and me moving out, I was was so scared of him I could barely face him for quite some time, and it is only now as an adult that I can hug him. I can't kiss him.
I am with Rap on this one, If I had wanted to go back, my mum should have used any means to make me stop... sadly it was the other way around, so the harm was already done.
Moving I don't think is a big issue. Sometimes a new school can actually be beneficial to a child (if they are the victim etc), it depends how all this is handled, because emotional upheaval is far far worse. I think this is what the OP wants to avoid, and why she thinks the Ex is a shit for suggesting something that will place the DC in an emotional dilemma. He is already faced with leaving his friends etc, now he is faced with the choice about gaining acceptance from his dad, but leaving his mum... This is BIG stuff.