Oh my God, that is really awful. You need help, lots of it, straight away. And youre entitled to it. If you don't take effective action, social services might well need to step in to remove your other children from danger. In the short term I presume you've locked up the knives and the other kids lock their door on the inside at night?
The police should have a specialist 'mentally disordered offenders' liaison officer, usually a sergeant or above, and in my experience they're pretty good. Even if you don't press charges they're likely to liase with the domestic violence team to get you a panic button etc. Adult mental health services and adult learning disability services might be great, or shuttle you back and forth unless he's already under one or the other (round here it boils down to having an IQ of 69 or one of 71... like it makes any real difference!)
The danger for all of you, and for his long term future, is that he is learning that knife attacks get rewards. And while I know you have kept everyone safe so far (no mean feat) and have few options, if he doesn't unlearn that now, he will be impossible to 'place' outside a secure environment when the day comes that he moves out of home.
I think what I'm trying to say is that you might need to look at all your options including leaving home for a placement, admission under the mental health act. Best of all would be trying for social services to fund a specialist residential college placement for young asd adults before he gets too old to be eligible, and winds up on a forensic placement merry go round.
I'm sorry if that's blunt, and I realise that most professionals and authority people will try to pass the buck, fob you off, minimise the danger.... Because it isn't really any one person or organisation's job, and they will have a lot of grief and aggravation with time consuming multiagency liason that will result in a very large bill. That's why the police can be the most use: they don't pay that bill and so have no reason to pretend you don't have a problem.
Pm me if you need to.
Hope things get a lot better for you all.