Hi chipmonk, it sounds like you and your son are going through a really hard time. And you sound like you're worried/scared/upset.........for him, and distressed and stressed......by the whole situation (?) which would be natural.
But if he has severe depression with psychosis he really does need some more professional help than he's probably getting by the sounds of it.
I'm guessing it isn't really helping by trying to help him "rationalize"/manage what's going on for him, so maybe the most you can do day-to-day, other than that, is try to just be there for him, try to be understanding, reassuring and supportive towards him.
But what you might be able to do is talk with his pdoc about what you're seeing/what he's going through. If he isn't seeing one right now though, then you could contact the last one he saw or have him re-refered.
It is really good that you sound so caring and to have his best interests at heart, just don't let any professionals brush that off and insist that they give/get him the help that he deserves. His safety and well-being are so important as you're clearly seeing.
And make sure that you're getting some support too, from friends, from family, online........wherever, I know it really can't be easy for you right now.
Alison
P.S You might also like to post this in the forum for Partners of People and Caregivers Support too, any help you can get would be good, right? Best wishes.