It's great that you've noticed that there may be a problem. My DH fell into the same category of being high-functioning. In fact, I hadn't really noticed that he was drinking because he hid it so well and never appeared to be drunk.
He finally admitted he had a problem to a friend of ours. In hindsight, all of the signs were there but I thought he was over-exagerating the problem. As a result, we decided to tackle it together without help, and he went cold turkey. We now know that this was an incredibly dangerous and stupid thing to do, as it can be fatal.
Luckily, DH sought medical help and also went to the AA. He is now on the 12 step programme and has been sober for over 18 months. I mention all this because my DH was also "high functioning" but since he stopped drinking the change in him is remarkable. He is a different person. So I suspect now that he wasn't high functioning at all, but "barely managing to keep it together" and not the brink of breaking down entirely.
The first step of course is for your DH to admit that he has a problem. Until he does, there is very little you can do. Like others, I found Al-anon very helpful. There is also a thread on here for the partners of addicts.
I quite often post on these sorts of threads because I think my DH is evidence that alcoholism can be tamed. He'll always be an alcoholic, but he no longer drinks. It can be done. Good luck.