It's just that being drunk once every 1-3 months just doesn't sound like an alcoholic.
I thought that too, but something troubling you has brought you to SR.
The standard units for men is 21 a week - a 500ml bottle of beer (5%) is 2.5 units and to be healthy this should be consumed maybe over 4 nights of the week (2 per night with a couple of nights off) Many men who aren't alcoholics drink regularly but they can stop easily and switch to tea/coffee etc.
Is it that you dont want him to drink at all? Is his drinking crossing a boundary that you have set up and him drinking crosses that boundary for you?