DP, who has three children from his marriage, is having a tough time arriving at an arrangement that he thinks is in his children's best interests (particularly one of them). He and his XW have a court-ordered arrangement, which is almost 50:50. What isn't working with this is that his eldest child (DSC1) has a very troubled relationship with his mum and is deeply unhappy at (her) home.
We and extended family (including the mum's own mother, to her dismay) have all witnessed the dynamic: DSC1 (9) is regularly shouted at, sworn at and physically dragged about by his mum, while the middle child is favoured and seems to be able to do no wrong (DSC2 is consequently developing some 'diva' behaviours that we're finding challenging). DSC1 desperately wants to live with his dad; he hates his mum, and has said as much for years. He cries almost every time he has to return to her, and begs one of us/another relative to not make him go, or to stay with him. He does seem to bear the brunt of his mum's frustrations, and this year at school, he has actually gone backwards, despite being bright. He is also developing toileting problems, so is clearly stressed.
DP, understandably, is finding this very distressing and ideally wants to remove his eldest from this situation altogether and into a consistently stable and loving environment, so his son can get on with his life and come out of this experience as unscathed as possible. (He'd consider having all three all/most of the time, but the youngest seems to like having time in both homes, and the middle favoured child ? for obvious reasons ? would rather be with Mum for now. Plus DP's work commitments, and therefore income, could become compromised at the moment with all three, with the younger two being much more dependent.)
The difficulty, as far as we understand it, is that the courts don't take the child's wishes into serious consideration until they're older (double figures?), and also, they seem to like to keep siblings together. We can both see the value of this in most families, but where DSC1 already feels loathed by his mum, DSC2 being favoured by her under his nose seems to be compounding how disliked the eldest feels by his mum - and making him resent his sibling. If DSC1 were to be with us all/most of the time, he'd still be with his siblings half the time and be able to develop his relationships with them then, but with Dad as the main adult, so all the children would be treated fairly.
We realise custody is a far-from-ideal course of action to be considering, but we can't see what else we can do. Mum doesn't think she's doing anything wrong but that it's all the son's problem, and honestly, we can't see her changing ? there doesn't seem to be any self-awareness there at all, or interest in changing the dynamic or building a bond with DSC1 (DP says there's never really been a bond between them). So DP feels that he has to try to do something for his eldest. Is applying for custody the answer? And how do you go about it, especially when it's for one child out of three? Or do you go for all three on the grounds of how DSC1 is being treated? Or do we have any other options in this kind of very difficult situation?
Sorry for the length, and thanks so much for any help with this.