Hi Helpless,
You are not really helpless, you know? It just feels that way because you are so scared. You are also resourceful and brave and strong - you made your way here and reached out for help.
I think it is natural to be afraid of honesty. Especially for those of us who grew up in alcoholic or dysfunctional homes (and I don't know whether you did or did not), we learnt to not feel and speak since when we did, we were emotionally abandoned. I also think that because you love your son despite everything, you don't want to hurt him. Being in the situation he is in, he will hurt hearing that he cannot come home to either of his parents. Hurting, unfortunately, is the pathway to growth, so for him to feel the natural consequences of his past actions is the only way he can learn and grow. Most lessons in life are painful and we cannot and should not guard others from this growth.
In order for you to not abandon yourself emotionally, it is important to practice the spiritual principle of honesty. We can be honest without being brutal. Al-Anon says "say what I mean, mean what I say, but don't say it meanly". We have to learn , to show ourselves that we are safe. That our inner adult will keep our inner child safe now. I feel you may be afraid of your son getting angry with you, abandoning you, not loving you. And that is coming from the place of the hurt child within. The parent inside of you knows he needs this lesson. Coming from your parent perspective, you have the power and the strength to say 'No, my darling, I truly wish things were different, but at this point in time, I cannot allow you to come home." You don't have to explain. You don't have to rub his past behavior in his face. He knows. He will get the lesson when you use the least possible amount of words. Only speak your boundary. Bless him, change yourself.
This will be difficult because it feels like we are falling , like the Earth will swallow us whole. Show yourself that those feelings are not facts. Show yourself that you, too will grow when you don't hesitate in the face of unreal fears and that you stand in the full power of your life.
Your son can turn his life around. It is never too late. Be the leader he needs and that resides firmly inside of you.