I am trying to draw on my own experience and of others, that I know, who have been in a similar situation. Firstly, if we are living around conflict, that does not help anybody's state of mind, stress, communication and relationship.
So planning your strategy is vital. It is important, not to be seen as reacting to the situation, that indeed you have a plan, and you have come to terms with the outcome of your plan. Of course, we would hope the outcome would be positive, and your son would agree to your terms. However, we know that probably won't happen.
So some things to do first, contact some mental health youth organisations, contact GROW. The mental health government system can and do have some good options available. They want to keep people out of hospital also, and there is a strong move to have support out of the hospital system. Find out about detox places; talk to a mental health trained police liason officer, hopefully there will be one in your area. If Police are eventually called in, it does help for them to know that the person has mental health issues. Tell all these people you speak to what is happening. Help could be just around the corner.
Doing all this will take time, (not as long as you would think) but being prepared is better than not. Knowledge is powerful and helpful. Carer groups are a source of much helpful information.
It is a harrowing time for you all, and it hurts to see the one we love confused, hurting, and not being very good to themselves, so it does help if you can live in different places, and have the space and time to relax and renew your energies to keep on supporting him. Are you in a financial position to help him find and live in other accommodation? Whilst he is living at home, he is not classed as homeless. Living in a caravan, couch surfing (staying overnight at different places), camping, living in a car, I am pretty certain are classed as being homeless. That means they can apply for public housing. However there is a waiting list, still better to be on the list than not.
I never lose sight of my son's possibilities, or the necessity and importance of my support and love that he needs to feel. That does not mean I agree or like everything he says or does. These are major mistakes they are making, however they do not have to be forever, they can turn their life around, many many have. Support in that change could come from some of the mental health organisations or the mental health system.
Keep having the conversation, I wish you well in whatever way you choose to go.
Fran