This is all quite simple if you break it down and look through to the medium term goals. It’s a game of short pain, long gain.
Your DS needs to hit is rock bottom.
He needs to feel the pain and consequences of his actions.
He doesn’t need a safety net in you breaking his fall by swooping in to rescue - so that he doesn’t experience the consequences of his choices to not work, take drugs and be abusive to others.
He needs a new path in life and needs signposting to get there. You can point him in the right direction to all of the available resources for addiction, emotional support for grief and trauma, to work and to find accommodation. These opportunities are there and it’s up to you to point them out to him calmly and systematically with encouragement but not to facilitate and enable his current existence.
I would assume his addiction is due to the untimely loss of his DF, I would assume he is in an toxic and dysfunctional relationship because he is living a toxic and dysfunctional lifestyle and I would assume that he would continue to behave in a toxic and dysfunctional way to you and your DDs.
So all of that explains his behaviour and you can look with compassion to the root causes of his addiction - but none of his lifestyle choices or behaviours are excusable or tolerable to others.
He needs to sink or swim - you can point out the various life rafts (addiction support, grief therapy, meaningful work or training, picking up with emotionally healthy friends, finding a place to live, getting out of a toxic relationship etc) - but he has to do this himself. Just keep reminding him that you love him but not his choices and you will always encourage and support him to heal his emotional wounds and rebuild his life - but not at further cost to his DSis or yourself who have endured enough of his behaviour already.