Welcome. I'm glad you found us and I'm glad you are here.
As you have now concluded, discussing his drinking with him does not make him stop.
His promises to you and himself do not make him stop.
Telling him his drinking bothers you does not make him stop.
Writing him a letter about how his drinking is impacting your marriage and family does not make him stop.
I believe by now you are seeing a pattern here.
He has told you he has a problem. He does not want to do anything about his problem. He wants to continue to convince himself he can control his drinking. Telling someone, "I have a problem," and doing nothing about it is a good manipulative technique. He's agreeing with you in order to get you to leave him alone about his drinking. And he continues to drink.
I am sure you are frustrated at your attempts to get him to see he has a problem. I would suggest you give Al-Anon a try. Until he wants to quit drinking, he will continue to drink. Regardless of how much it bothers you.