“I’m not a drunkard, I just drink”
Alcoholism is progressive, it has a beginning, middle and end. It sounds to me like your brother is at the beginning stage. He is an HFA (high functioning alcoholic). He does everything he is supposed to do and drinks the rest of the time. He could kind of plateau in the beginning stage for quite awhile but eventually he will go into the middle stage. This is where he will get sloppy, problems will be more apparent; missed work, money shortages, relationships messed up, etc.
It is really tough to talk to these guys. At this point it is hard to show a downside. The alcohol is still more helpful than harmful. A drunk can quit at any stage in the process but the beginning is by far the hardest.

That is a lot of jabbering. The point is that most likely your brother is already an alcoholic. Which means he is a path that is going to become progressively painful. For you, the people who care, and for himself.

This is speculation but:
I think the guys who have all the relapses are the guys who try to quit in the early middle stages. These folks have consequences for their drinking but the payoff is worth the pain. I think that is what makes them go back, even when they know better. That and the fact that they are usually younger and don't like to admit defeat.