Unfortunately, that is something he is going to have to see for himself. If his behavior is such that you don't want to be involved, then leave him to find out on his own. Otherwise, be prepared for things to get worse and not better if he truly has an issue with alcohol. Probably the worst thing to do is to excuse or rationalize any behaviors. On the other side of the coin overindulgence or berating does not help either. Just be honest about your observations and feelings in the matter and let things be.