Just one glass would tick me off. I quit once for a month went out to dinner with my wife and ordered just 1 beer. That sparked the obsession I thought i'll order just 1 more. On the way home i grabbed a 6 pack and drank draily from then on again well over a decade went by before i ever quit.
I also was not an alcoholic either for the first eyar sober I had a lot of problems but alcohol was not one of them. It was not till i was sober a year that I realized gee i'm an alcoholic *sigh*. And sitting around an aa table that first time after a year of being sober it was well sobering. I thought how the hell did my life end up here? I have no idea. I just drank and drank and one thing led to another and there I was and thats just the way that it is.
I think its a very slipperly slope for him to be drinking. I think he'd be playing with fire and would likely get burned sooner or later. But knowing how I am nothing would get in between me and my drink! So my advice to you is it could be a bit of a battle for you till he realizes that he's an alcholic assuming he is one and decides for himself enough is enough.