Nothing to do but to smile, bite your tongue and watch this play out. He's got some further adventures to endure before being willing to do much of anything to effectively help himself and calling him on it will make him defensive, upset your home and not change a thing. You don't want to deliver him a gift wrapped excuse to shift the blame onto you when he starts up again.
Low alcohol beer should allow him to coast for up to another 6 months, figure some dope smoking in there at some point if he's so inclined.
At some point this year he'll want to talk about how good he's been for so long and how he is going to be very careful this time around and do so much better that you'll be amazed at how responsibly he'll drink and that he's now made a few test runs and they've turned out incredibly well so you should take the news about this very calmly and be happy for him.
It's coming from a universe you can't understand, and it's really futile to try.
When he says those things all you can really do is nod. The next phase of your mutual adventures has already then begun and there is never a turning back of the clock.