Thanks for all of the helpful posts!
Apparently there's a 3-4 week waiting list for the rehab clinic that he's trying to get into, and he has to call in sober every day. I'm also debating whether I should let him merely sleep on my couch until he either gets in or gets rejected, and make it clear that he's on his own after that, or leave him on his own entirely, both before and after rehab. Thoughts?