Lucy, welcome. You've tagged onto an old post and you will have more responses if you cut and paste this onto a new thread of your own.
I am sorry you are going through this with your son. Many of us here have grown kids who have similar issues and make similar choices to live on the fringe of society. There is nothing you can do. He is a grown man of 26 and he gets to make whatever choices he wants regardless of your feelings or what you want.
There is a great article at the end of my post here on detachment, it would be helpful to read it. Detaching from our kids is sad, it is hard and it takes a real commitment to override those feelings of being responsible for your son no matter what. Your daughter is correct. There is nothing you can do, only he can make the decision to change.
Here in the states we have NAMI, the National Alliance on Mental Illness which provide resources for the parents of mentally ill people, perhaps you can look into something like that for YOU. Once we recognize we have no control over our adult kids lives, we can start to put the focus back on ourselves. Most of us need as much support as we can get, therapists, parent groups, 12 step groups, Family Anonymous, whatever it takes for you to find someone who can support you, have compassion for you, offer you tools to help with detachment and guide you through.
At a certain point we have to stop enabling our adult kids and learn how to detach and accept what is. Your son is aware that it is cold and raining, he is CHOOSING this. You can't change it. You can ruminate about it and have it impact every moment of your life, which will ruin your life, or you can begin to learn how to let go.
You can learn to "do nothing" as we have on this board. Doing nothing doesn't mean you stop loving your son, or praying for him or wanting his life to be safe and happy. It means to stop giving him money, stop doing for him what he has to do for himself, stop doing more for him then he does for himself, stop worrying about his life when you have no control over any of it. We have to learn how to take the relentless focus on our adult kid and put it on ourselves. That shift is particularly hard on us parents.........but for his sake and for yours.........you have to learn to let go.
If his choices lead to his death, then that is his choice, you cannot prevent that outcome. Our kids are remarkably resourceful, manipulative, cunning and quite smart, they usually land on their feet and live the life they have chosen to live. We stay up late horrified and full of fear for the choices they make and yet, they don't suffer in it, we do. With support, understanding, empathy and tools, you can learn to stop suffering about the choices your son is making and rejoin life and in fact, enjoy your life.
This is hard so please find supports. Keep posting, it helps. Read books like Codependent No More by Melody Beattie.........., attend a Codependent's Anonymous 12 step group.............change that focus onto yourself. You will get through this..........we're here...........hang in there...........wishing you peace.