|Since you have worked for this individual for over 12 years, your relationship is presumably strong enough to permit you to speak openly and candidly with him. |
The problem is that although you would like to make him understand the damage being caused by his drinking, he is probably aware of it. But, he has probably consumed a rather large dose of denial as well.
Your boss does not need information. He needs help. Although you want to tell him your concerns, you need to tell him more than that. For example, tell him the names of some local health care providers who specialize in this field, or tell him to attend an Alcoholics Anonymous meeting.
You can most likely give your boss a rational explanation as to why he should not be drinking on the job, but he needs to deal with the underlying issues that are causing him to do so. That calls for professional help, and that is the sobering truth.