I think asking him to quit is perfectly reasonable given his history.
I do not know the guy at all obviously, but As are great liars and hiders of their use and abuse of alcohol and drugs. It is pretty uncommon for someone to be on alcohol, coke and meth and just cut out the drugs and drastically reduce the booze. I would be very wary of this.
In terms of him having everything "under control", I know a lot of people like this, that had a reasonably manageable life, until they lost control. That loss can come quickly and hit you hard.
Good luck and keep posting.