in my opinion it is time to make him leave, even if you can only bear it if you pay for a place for him to stay. Kay took more drugs when younger than now, which is one reason we wanted her marriage to work and for her to leave. So we bought her and Lee a house, a stupid decision that we regret, but funding a cheap apartment may be worth it for your peace of mind. You can also just evict him and pay nothing, but I don't know if you are able to do that yet. We were not. But funding Kay did NOT help her. However it helped us. At least she wasn't using around us and her siblings. That made a difference.
Do whatever you can do to separate your living quarters from his. You already know it likely won't get better anytime soon and that we are all powerless over our beloved children's choices. Living with us doesn't change them. Its time to consider changing your relationship with your son. Sure, you love him. We love Kay to the moon. But we don't like her and she can't live with us.
You could get arrested because of the drugs he keeps or uses in your house. Pleases seek out Al Anon tonight, if you can.
God bless.