Learning to accept

My 19 year old stepson has a complex mental health diagnosis, addiction, and childhood trauma. He is currently living with us on a temporary (15 months and counting) basis.

We have issues with being able to set and maintain boundaries when we really don't have any way of enforcing consequences of any kind. We have agreed a basic set of conditions for him to live with us, but when those conditions have been broken - e.g. no coming home after drinking because of the risk of aggression, damage and violence - he walked out and ended up either sleeping under a bridge for a couple of days or getting admitted overnight at the mental health ward.

His mother is trying to get him hello through the NDIS but in the mean time, continuing substance dependence means staying "a few weeks" had turned into 15 months, why the likelihood being another six more.

Is there anything we can do to set limits without ending up in Emergency for the night? Do I just need to learn to accept that he sets the boundaries?

I know I'm probably catastrophising a little as it's the anniversary of the passing of a relative who was in a similar situation to my stepson, on and off, for fifty years...