Please do not give him a letter like this until you are absolutely, 100%, completely and totally ready to follow through and leave. Do not give him this letter if you are hoping that these words will be enough to change him, but won't be able to leave yet if they are not.
Because I have to say, in my opinion, they probably will not be enough to change him. You have to prepare yourself for this in every way, if you decide to give him this letter. This is the most important thing. More important than what the letter says.
However since you are asking...I wonder would any of this be news to him? Have you never said these things before? Does he have no idea that his drinking is a problem for you?
And is "considering getting help for quitting drinking" really what you want from him? Or do you want him to quit? If that is what you want, you should say so and then let him figure out how to go about doing that. Your concern for his health is all well and good but it's only his concern for his health that will ever get him to a doctor.
I'm not a big believer in ultimatums, for the most part, but if you are going to give him one, give him one that would really make a difference if everyone followed through.
I'm not a big believer in ultimatums because I don't believe people really understand what losing something means until they have actually lost it. Which would mean he was making changes in order to avoid punishment (in his eyes) instead of really wanting to change his life (the only way something sticks -- again, in my opinion).
So there's all that for what it's worth. I hope this goes the way you want it to.