Too many people here speak only from their own limited life experience and project how they feel about their own little darlings who do not behave in this terrible way and then criticise someone who is going through a hell they will never experience.
Ignore them.
I am a retired nurse and social worker. I have worked in community teams dealing with children and adults with learning disabilities, many of whom had extremely challenging behaviour.
My advice is this:
Find out the name of the head of child services in your area and write a letter to them making it very clear that you fear for your own safety, the safety of your other children and for your son’s mental and physical health.
Give examples of his aggression towards you, the theft, the vandalism, the influence on the younger children and the effect on the overall household.
Describe the effect on your health and the health of your younger children.
Describe how he interferes with and undermines your parenting of the younger children and the effect of this on their behaviour.
Specify your complete lack of support and isolation.
Point out that you are being left to deal with a young person who, if in any type of supported care, would be assessed as needing an entire team and quite possibly a 2:1 staffing ratio.
Describe how your son refuses help in the community such as CAHMS leading to them abandoning him and that he refuses to attend school.
Point out that at age 16 he is fast approaching adulthood and you, as a single woman with younger children, are being bullied, threatened, assaulted and effectively held hostage in your own home by this young man.
Tell them that after years of asking for help and getting nowhere they are now personally on notice that this is a tragedy waiting to happen.
Tell them that although you love your son you also fear him and you fear for your younger children and because of this you cannot continue to have your son living in your home.
After finding out other relevant names copy this letter to the chief constable of your area, the head of education, the head of CAHMS, your GP and your MP.
If you can possibly afford it, send all letters by registered post so they have to be signed for.
Also contact your nearest women’s refuge and speak to them about your fears for your safety. You won’t be the first woman to contact them who is frightened of a teenage son and they might be able to add some weight to wake up local services to your situation.