Part of what you're up against is that most people are clueless about alcoholism. Your ex is working and isn't a Thunderbird-guzzling vagrant, ergo, not an alcoholic, especially to other drinkers. For them in particular, if he's an alcoholic, where does that put them? So it must be that you are overreacting. They also believe that all he has to do is "cut down," which he says he's doing, so...
And in all breakups, people want reasons and they take sides. Your ex is running around playing the victim to everyone and since they only know his version of events and haven't seen him at his worst, they believe him.
It's rough, it really is. I feel for you. In time, if he's like 99% of us, the real extent of his problem will become obvious to others, especially as you're not around to cover for him. Of course, at first it will be blamed on the breakup. Sigh.
Hang in there. About all you can do is say "you don't know everything that happened" and leave it at that, if you have to say something.