Mine went through a patch of this. Not as severe. And at 9. In trouble every day, sitting in exclusion every day. I had days where I hated him. Genuinely could have throttled him out of frustration and exhaustion. He was a badly behaved, smug, entitled little sod.
Firstly, the school were fucking useless. Made everything worse in my opinion. I let that carry on for far too long.
What worked for us, was firstly, taking him out of school for a week. Without their "straight to exclusion" as soon as his name was mentioned, he didn't feel like he was doomed from the start, so what's the point in even trying. We've since changed schools and it's so so so much better.
Whilst at home for that week, I became the proverbial iron maiden. He'd lost all respect for authority including me. So I ran the week like an army sargeant major from the 30s. It was hard, it was horrible, but it worked.
I'd get him up. He'd do the work I'd collected from the school, under my glaring eye. He was allowed no TV, no games consoles, no tablet. If he wanted something to do, he was given the hoover, a duster and some polish.
The first couple of days were awful. He yelled, he had tantrums, climaxing in him saying he was going to punch me. At which point, I bent down in front of him, put my face right in his, stared as coldly as I could right in to his eyes and very quietly told him "Try it. See what happens." I meant it as well. I don't know quite what I would have done if he did. But what mattered was he knew I meant it, and it was like the penny dropped that finally there might be a consequence to his behaviour. It was like in that instant he realised he didn't own the place, in every sense.
Things changed after that. He'd listen. The rest of the week was still boot camp for him, but he just got on with it. With no other distractions, he talked. About why he did things, about why he felt he could, about how he was going to accept the help to change. I told him what I was going to do to make things better, I promised that we'd find a new school. I also told him that I'd keep him home for as long as I wanted under my militant regime if he did not put the effort in. And he really didn't want that.
A huge part of it, was the terrible influences of the children at his school. A tiny minority, but they were unfortunately the ones DS was impressed by and gravitated too. Their parents were unconcerned about what their children were doing as long as they were out of their hair. Getting him away from that is key.
DS is now a very accomplished boy. It seems incomprehensible that we were going through this under a year ago.