Please consider starting your own thread to discuss your son's issues, so as not to detract from the original poster's need for feedback on their situations.
Next, I am going to be blunt and I am sorry if it is offensive, I do not mean it as such, but sometimes outsiders can see things we cannot.
Your son is a danger to himself and to you. He is violent, and you responding with violence in turn is not helping the situation to say the least.
You seem to have every excuse under the sun as to why help is impossible. It is only impossible if you make it so.
If you continue as is, you will end up in the newspapers as a tragic situation with one or both of you dead. In other words, things cannot continue as they are.
What are YOU going to do about it? Post here and seek sympathy from other parents of problem children? Wallow in your misery? I understand we all do that to a certain extent, but what ACTIONS are you going to take to improve your life and that of your children?
Canadian law in many ways is similar to US law. We also have members from Canada who hopefully will jump in with advice.
First, contact child protective services. Tell them your teenage son is violent and you are in fear of your safety with him in the home. Tell them he is mentally ill and refuses to seek psychiatric care. Tell them he cannot continue living with you as you are unable to provide him the care he needs as a mentally ill minor. Make them do their jobs and find him a group home or residential treatment center - or jail.
The next time he strikes you, call the police and immediately press charges. Tell them you want him taken to a hospital and evaluated for mental health issues. Don't do this yourself, make the police do it. That way if son lashes out it will be plain for all to see how sick he is.
Secondly, contact your ex wife and tell her you cannot handle him anymore. Perhaps you can take younger son and she can take older or if she is not willing to do this, once again CPS can get involved and find him a placement somewhere else.
Don't think it will necessarily be so easy to put him out at 18. He may have tenants' rights and you may have to forcibly evict him using the courts. For this reason as well as the others I have mentioned, seek legal advice.