I've said this before but a boundary isn't the words we use to communciate it, or even if we communciate it at all, it is the action we take to keep our life sane and happy.
you don't have to tell him at all. Most people "get" that putting their family at risk, risks the family staying together. and perhaps you may have mentioned how you feel about his drinking before once or twice?
If you want to communicate your boundary, as you say, however you word it - he is unlikely to welcome it respectfully with an open heart, so don't fret about the words.
In the end an ultimatum is one way to a communicate a boundary, it is just the final (ultimate) one: and there's no problem with that as long as you intend to follow through and the action follows.
The only problem with ultimatums is if you keep making them and not following through. ultimatums or any boundary don't "make " someone act differently, they define what you will do to keep your life sane and happy.
good luck.