I know you are scared and I feel for you. I am always struck by your posts because your son sounds so similar to mine.
My son has been back at home for a year and we had a period where he was drinking up in his room and getting drunk. We had a few confrontations when he was drunk and that did not go well. He said he was depressed and wanted to kill himself . I feel very wrong in saying this but it seems that its a thing he says when he is not sober and has been caught in our home. He created this big awful chaotic situation about 3 times and then when sober in the am, acts like nothing has happened. The last time this happened about a month ago, he admitted he needed help and promised to get some. We made it clear we would help him and offered many ideas but he ultimately we are here and he hasn't gotten help. Yet, the drinking seems to have stopped for now. I believe that he self medicates because he is lost in what he is going to do in his life. All of his peers chose the path of college and moved to campus. He has alienated himself from all of his past friends and his path is a lot less clear. Thankfully, he has a very good job and seems to like it as far as jobs go. He does it because he knows he has to have money. He gets up everyday and works a full day and that is our big reason why we continue to allow him to live at home. Also, he goes through these periods of messing up but always pulls himself out and does a bit better the next time. As long as he is moving forward we try to remain supportive.
I thought I'd look into the online therapy option for someone for him to talk to. Curious if your son goes to therapy?