I have no idea if this is the right place to post or not but it felt appropriate. Apologies if not.
DS1 is my only son with my ex. I have 3 younger DC with DH, who I have been with since DS1 was 1.
DS1 has always been incredibly difficult, since he was about 2. He was diagnosed with ASD (very high functioning) at age 5.
His behaviour has continued to escalate over the years and last year, we decided, as a family, that he might be happier if he lived with his dad. He has always had regular contact with ExP and we have a very amicable relationship. In the end DS begged us not to make him go there and of course we let him stay with us, I'd never force him to be unhappy. We said he'd move to dads for secondary school, but finish out primary here. He wanted to go up then, he likes the look of the secondary near his dad (though he also loves the taster things he's been to at the one here
) He's now end of year 5, so only one year left. He promised he'd stop the violent, nasty behaviour to us and his siblings and for a few months all was fine.
Despite this, this year his behaviour has become out of control. He breaks doors, carpets, furniture etc in rage. He throws stuff around, hits and pushes his younger siblings, screams, shouts and swears at us. He follows people around making irritating, repetitive noises to wind them up. He rages at everyone and refuses to do as we ask (can you put your rubbish in the bin, can you pick stuff up off your bedroom floor, can you get your shoes on please) Basic stuff, he's not Cinderella. He steals - a lot. Toys, food (people's personal special food, tge for a cupboards full he can have but he chooses to take the chocolate coins from a siblings party bag). His triggers are always either: being asked to do something he doesn't want to, or being told he can't have/do what he wants. This is just parenting isn't it? No you can't go out in the pouring rain. No you can't walk to school today, you won't get there in time. Ask earlier tomorrow.
This culminated in an incident at school in which he pushed his younger brother over, who sustained a fair few cuts and bruises. This led to consequences at school and at home (no iPad here, a report card for a week at school). He kicked off, big time, and at one point tried to grab a knife. His father said that enough was enough and he'd take him to live with him, has applied for a new school etc. He lives to far away for shared care and has refused to move back. We still live in our original home area.
DS is distraught. He desperately doesn't want to leave school, he is happy and thriving there and I'm loathe to do it, but he patently isn't happy here either. He's convinced we treat him differently to his siblings, and whilst he's correct that he's told off more often, that's because he behaves this way more often. His siblings get exactly the same parenting if/when they are badly behaved too.
So now I'm stuck in this situation where I feel like I can't help him anymore. We've tried children's services, CAMHS, school (who are great but can't offer much), GP and so on. Nobody wants to help him. He has serious anger management issues. I don't want to lose my son but I feel like I have no choice. I want him, and my other kids, to be happy and feel safe. My DS2 is close in age and really very negatively affected by all this, it's very upsetting. Why has nobody helped us? Why am I being forced into 'sending' my first born away from me? I utterly adore him but his behaviour is destroying the rest of us in our home and family. My husband is at breakdown point as he gets the brunt of it, by virtue of not being biologically his father. No other reason. We've tried everything we can - even my DM had him for a few days and tried to get through to him, he came back and kicked off the day he came home 
I just want him to be happy, and everyone else too. He knows I want him here, he knows I love him. I've explained, and he does understand, that it's not him that makes me feel so stressed, it's his behaviour, but he just can't not behave this way. Typically, he doesn't do it for his dad, but as he's only there every other weekend I suspect it's a bit 'Disney dad' and he will if/when he's there more permanently.
How do I fix this? How do I help all my babies?