Sorry this has haappened, but the school didn't have much choice. It's nobody's fault but they're not meeting his needs and he has a pattern of dangerous unpredictable behaviour.
I haven't been through anything this serious as DS got help much younger so we were on top of things younger. When a kid has reasons for bad behaviour and it's been happening for a long time it's easy to kind of pass over just how dangerous they have become to themselves and others. You can get habituated. So I would just say that your son is very disabled and yes, quite dangerous. Good idea to check for ADHD as well as ASC; kids with both diagnoses often have an extra-hard time.
And IMO @Venetiaparties advice is mostly along the right lines, except for just the very first bit:
I would tell him this is precisely where he will end up in less than twelve months unless he stops this behaviour immediately.
I wouldn't do this bit, not if a kid has an ASC, for two reasons. First, because he probably wont know what you mean by "this behaviour". We do, you do, but
ASC often comes with not generalising and not making connections. So he didn't bring a knife to school again but no matter how clever he is, he really might not get "don't bring a knife to work" or "don't take a cosh to the shops" as well. And second, because the anxiety of not knowing how to avoid this terrible future might actually make him worse. Anxiety fuels aggression, self harm, and all kinds of things you don't want. So the other suggestion upthread (sorry I forget whose) of some calm-down time before you intriduce other activities may also be a good idea.
(Oh and also you mustn't tell a kid with an ASC that they will end up somewhere in precisely 12 months time because if it's 11 months and 3 weeks they'll think you lied to them
)
But apart from that - yes to the charities, yet to Princes Trust, and also yes to the youth offending team. The aim of the YOT is to keep kids out of prison. He wont be the first lad with developmental issues that they've seen. And he has one thing in his favour - he has his parents' support. You can back them up, and him up, and that will all help.
we've worked it out so that he is leaving voluntarily hence no record for him nor the school.
I am not sure how much diffference it makes at his age but that may actually stop him getting some kinds of help. When kids are permanently excluded there's usually an obligation on the education authority to find something more suitable. And it adds to the picture of how severe his needs are and how they aren't being met. If he just leaves voluntarily no-one has to do anything and they can shrug him off.
Good luck OP, it's a minefield.