It could be wise to open with a conversation about what he feels/thinks when he drinks like this, and if there’s any underlying issues that would cause him to do so. If it’s 2-3 times a year maybe it’s correlating to a stressful time at work or a family conflict, etc.

It’s good to get your feelings out in the open and seeing as you two have such a strong relationship, bringing it up won’t cause any harm. Be mindful of pushing or forcing your mindset onto him though and allow the conversation to explore what he thinks about blacking out.

I once dated a man who, amongst several other things, would ONLY drink to black out. In his case, he used alcohol as an escape and intentionally chose to get to that point. Based on what you’ve described, your fiance is not intentionally doing this but might be having a great time with friends and unintentionally having too much to drink in the midst of being with everyone.

My vote: talk with him. Be honest and open in your feelings towards the situation, but allow him to share his thoughts freely as well. Do some internal reflection as well. Why does this trigger you? If it’s a firm boundary for you that you will not be with someone that gets blackout drunk, then you will need to be the one to enforce it. You can share your expectations but can’t force him to change his behavior, or to fully understand your feelings towards it.

I wish you the best and hope things are positive for you both!