I have a lot of different points to make and questions.
Firstly I want to say that all behaviour is communication. So his behaviour is telling you something. What it’s telling you will need unpicking.
What’s his relationship like with his sister. Do they get on? Have they been cast in roles, her the golden child and him the black sheep?
Are you disrespectful about mum to the children? Do the children know you think she’s a crap parent? It’s good to label behaviours which are not acceptable unacceptable (her hitting) and get the right help, however bitching behaviours are divisive and undermining, they will create issues.
A combination of PACE with fair consistent boundaries delivered with warmth and love is probably best.
what exactly are CAHMS doing? This is a seriously underfunded ineffective public service. What are they actually doing with him? Weekly counselling? Has he seen a psychiatrist for assessment?
Do you have access to family Heathcare through your work? Can he have a physiatrist assessment if he hasn’t had one. Also weekly counselling if he’s not receiving this.
How does he feel about school? Why does he avoid attending school? What does he like and dislike? What would he say the biggest issues are?
Hes clearly got into a cemented routine behaving as he is in school. He now has a reputation among pupils and teachers. He will be fulfilling expectations when he misbehaves. Would a change of school help? A fresh start if he wants it? A clean slate? Alternatively under his existing school there should be access to a PRU (education in alternate setting) or online learning. Speak to the school to find out what they can offer and if fobbed off ask these questions to the LA directly.
What does he do out of school?
what are his interests?
Has he positive male role models outside of school?
Has he had any professional careers advice? Has he got plans for the future? Plans for an apprenticeship maybe if he’s sick of educational settings?
You say he is a leader of men, this is a top skill and could be put to great use in a club or hobby setting. Could he volunteer to help younger kids groups at his boxing gym? Or other sports clubs? Giving responsibility can help growth.
are there new things and clubs he would like to try? Climbing indoors, canoeing, chess?
Is there a Saturday job he could take? One where he becomes part of a team, is valued and becomes an essential cog in teams performance?
How’s his self esteem?