In my experience once you set a boundary you have to keep it. That's why its best to know that you will before you say it.
If you don't keep your boundary you are setting yourself up for this merry go round again.
His decision to get better has nothing to do with you or the kids - its up to him only. Don't get pulled into being guilted that sobriety somehow has something to do with you it doesn't.
Keep posting - others will be along with more advice.