I went through this same sadness with my son and in the end he used when he was at home and he used when he was not at home. The hardest thing I ever did was to make him leave, but it was also the best thing for him and for me.
There comes a time when we need to give up our front row seat to addiction, it's the worst seat in the house and serves us no purpose.
We can't drag them into sobriety, but they sure can drag us down to their darkness and it's a nasty place to be.
What helped me was to go to meetings and surround myself with support while learning to work 12 steps that would literally save my life.
Your son and mine are both in God's hands and we just have to have faith and hope that one day they will find a better path.
Hugs