I'm glad that this has worked out well, womanwholivedinashoe - and I'd like to reassure you (as I said in my first post) that moving schools at your ds's age can work fine - it did for us - and in fact has greatly improved the dses lives, in ways that we could not have anticipated when we moved up here.
Rapaccioli - I think I didn't express myself well to you. I absolutely understand and respect why you didn't want your older dc to go and live with his father, and when I was talking about children making wrong decisions, and it being a parent's responsibility to pick up the pieces, I wasn't clear that I wasn't specifically talking about this situation. What I should have said that as children grow up, we as parents do have to let them make some decisions, and this must increase as they get older, so that they can learn to be independant when they leave home. However, the level of decision they should be allowed to make has to depend on their age and maturity - for example, I let my dses go without coats to school when in Junior school (I didn't nag them to remember them) and if they were cold, they had to live with that consequence.
As they got older, I let them make decisions about things like music lessons, scouts, other hobbies etc. When it came to time to move to secondary school, we listened very closely to the boys, and supported them in the choices they made about which schools to apply to, and whether to take the 11+ - even though ds1 and ds2 chose, and got into, two different selective grammar schools, when it would have been far more convenient, as well as more economical, to have them both at one.
However, there are clearly some decisions you can let a child take on their own, and others where you as the parent have to be involved to some extent - and in the decision that you and your older dc, and the OP and her ds had to make, clearly you and the OP had to be heavily involved. I don't know what discussions went on with your dc before the point where you told them if they went, they wouldn't be allowed back - I assume you were absolutely clear about why their 'father' was a bad parent, and why it was such a bad idea that he moved in with him. Assuming this had happened, and not worked, I suspect, in your shoes, I would have forbidden him to go, instead of using manipulation as you did.
And I still find it really hard to imagine any mother turning their child away - even if he had broken up his family, gone to live with his father, and deeply hurt his mother and siblings. I'm imagining a situation where he's made this (wrong) decision, deeply regrets it, has fallen out with his father and feels rejected by him, and literally has nowhere left to turn because his mother is rejecting him too. I could not do that, and find it hard to imagine anyone could.