(((Windblown))) - I understand that "doing nothing" goes against everything you feel, as a mom. Unfortunately, if he wants to keep using, he will do it right under your roof. Having him home is not going to prevent anything. I'm saying this as both an RA and recovering codie.
There was a 19-year-old kid in my old home group. He had 4 years clean and sober...yep, realized he was an alcoholic when he was 15. His cerebral cortex hadn't matured, either, but he sought recovery.
We A's are very resourceful. Honestly? I wish I'd been faced with consequences when I was younger as I might have sought recovery a few decades earlier.
I'm so sorry you're going through this, and am sending extra hugs and prayers for you all.
Amy