This was a very good letter. You stuck to the facts and reality of the situation. Not sure if a letter is enough, you may have to say this (and should say it) directly to him if it is safe for you and possible to do so. Maybe with someone you trust also there for support.
If he tries to counter this letter (and NOT get help) keep saying the exact same thing, esp the part that the marriage with his drinking is over. That was a powerful statement.
Best of luck.