If this is the only red flag and he's not an alcoholic, he went out with friends the day before the wedding to celebrate, maybe you're making a too big of a deal out of it.
Not knowing the entire situation, how long ago did you tell him your ex did this, how did you tell him?
How is he as a person? Is he the kind of person who goes out and thinks I'll just drink a couple and I'll be fine and end up drunk?
How are the people he went out with and how is their relationship? Would they get him filthy drunk, would he be one to not be able to say no to shots, to celebrate the wedding?
I feel there's so much of this story that's unknown from what you said that nobody should draw any conclusions that don't conclude a couple of huge IF's.
In many Western cultures around the world, men would find it hard to tell their friends they won't drink alcohol the day before the wedding. In many cultures he would have been part of other celebrations where he cheered for others to drink excessively before/during/after a big life event. If he wants to drink, I feel it's wrong of you to tell him not to because that was one of the things your horrible ex did, while not that horrible on it's own.
It's more than okay to tell him not to repeat bad behaviour, getting drunk the night before the wedding might be completely fine. Like I said there's a lot of reasons why it could be that it's not okay, depending on how long, how many times and how seriously you emphasised it would hurt you if he got drunk vs you told him not to do it.
Relationships aren't perfect, in my 35 years I've seen a lot of couples, been part of two long term relationships. I only saw one couple I thought was perfectly fine, great together. Most of the time compromises have to be made and as long as both sides are compromising the same amount (and of course no abuse and a lot of open communication) then you are a lot better than most other people. It's important to be happy too, of course, but some people keep things bottled up and are mad and unhappy all the time for things that could be cleared up with a conversation. Not that it would change the past, but it could affect the future and talking about things is incredibly helpful.