I'm not sure where you get the idea your dad isn't an alcoholic, but it really doesn't matter what label you put on it. It is what it is--alcohol is messing up his life. It's already destroyed his marriage, and as others have pointed out, he could kill himself or someone else while driving drunk. Think that might be a bit of a problem?
I suggest you look at
this thread, which is "stickied" at the top of the Alcoholism Forum. "Under the Influence" provides some very helpful information about alcoholism (or, if you prefer, "severe drinking problem").
The bottom line is that if someone doesn't see it as a problem he won't take any steps to deal with it. I know you want to help your dad, but helping him to excuse it and rationalizing it away isn't going to help him.
You can't MAKE him stop until he is ready, but you CAN arm yourself with the facts and let him know that you love him and will support him if he decides to stop his downward spiral. Untreated alcoholism (or "severe problem drinking") inevitably gets worse, not better.