I'm so sorry you are going through this. None of us know what to do.
My question is, is there an assisted living facility or other supportive place he could live, somewhere with supervision and companionship?
Do the doctors know he plans to stop taking his meds? I would ask for injectable anti-psychotics.
Do they know that his injuries were caused by severe psychosis? I don't understand how they could send him out without assurance that he will not hurt himself, but I have seen my own family member's journey through the system and that is exactly what happened.
There is a chance that if you tell them he can't come home that they will have to find a placement for him. Just explain that you cannot watch him 24/7, that he ran away and you could not get him to come home, that you do not feel you are able to meet his needs for safety and supervision. Also that he plans to stop taking medication.
Where we live, if we refuse to bring our family members home for issues of safety or that we do not feel they are well enough to be left alone, sometimes other placements are found. Sometimes people are discharged to the homeless shelter.
Is there a social worker assigned to your son? Talk to that person. Request a social worker if there isn't one.
Find out whether your state laws http://www.treatmentadvocacycenter.org/browse-by-state allow you to petition for court-ordered outpatient treatment. A state social worker or local crisis line might be able to help with this; they do (somewhat) in our state.
I am also in the position where something terrible happened "during my watch." Now there is court-ordered treatment and a treatment team; they're all great and it's working for my family member to live at home, but I could not do it without the court and the team. I already proved I couldn't.
As far as your own depression, counseling or medication or a combination of the two might help you. I hope you have the time and resources to seek care and help for yourself.
Everyone told me I did everything I could for my family member. Yes, and it didn't work, truly heartbreaking.