Marsey,
Divorce takes a very long time, I am not sure why you are waiting. He blew 5 months of sobriety and didn't care. Also, I am not sure why you would want to tell him, he is just going to dodge the person serving the papers. I am sure you warned him more than once that if he drank you were filing. I don't think you need to be lovey dovey on why you filed. FILE!! Let it fall where it needs to go.
You are not his babysitter. If he was concerned of your plans, he would have stayed sober. Do what you threatened, as I am sure that he doesn't even think you have the guts to do it. But only do it if you have the strength to follow through. This is not a game, only do it if you plan on doing it. Lots will change in the next 6 to 10 months that he could "show" you his sobriety, but nothing is ever going to change. wash, rinse, repeat!!
Hugs my friend, you deserve better!!