I do have a few friends who had similar situations - big group of friends who were into clubbing and large houseshares (which meant house parties & a fair amount of drug use of ketamine, pills and less frequently, coke). One couple had a suprise pregnancy, moved out and both left that lifestyle behind for a few years. I have known them use recreationally since, they're still quite 'alternative' - but it's with strict parameters, was only when the children were older, and frequency was probably once or twice a year, as it was at festivals. They were also much more careful about what and how much they were taking. Their partying is equal - each gets a turn - which is why it was when the children were older, ie old enough to stay with family for a weekend.
Two other friends became pregnant from that group, one of the boyfriends thought he was going to carry on that lifestyle and did during the pregnancy but he did change when the baby was born. They're not together now, but he is a hands on dad. Once baby was here I think the responsibility hit him. The other couple it didn't work out and she was a single parent from the off, he was not ready to be a parent and going out with mates was still far too important to him. I don't think that was particularly about the drugs - he wanted the single lifestyle of going out with mates, consequence free. No excuses for him, if he wasn't ready to be a parent he should have used contraception.
What stands out to me about your post is not so much about the drug use but his expectation that he can still keep part of the lifestyle of being young and not having responsibilities once a month. And his expectation that that's ok. For parents I do know who use, there's reasons it's so rare (eg festivals) - the additional risk to health (not just from the drug itself, but the unknown, eg what its cut with) the financial cost, the practical difficulties of sourcing, eg because of being more risk averse about buying (no longer being prepared to buy in a club, not risking dark web/posting to home) and the amount of time it takes to recover (feeling fluffy headed for a few days is not good with a young child)
What's difficult to know from your post is whether it's because he's naïve about what is realistic as a parent, (which many people are about all sorts of things, like their social life, gym, sleep etc) or whether it's because he's actually going to put this lifestyle first. Unfortunately you and he might not know that until the baby is born. And you do need to think about how you would manage as a single parent because if it is the latter then it's not ok.