I actually think, reading more OP, your problem is you have mothered your DP for far too long - you have managed the finances and dealt with bills, so he's always been able to see his money and his time as his to do with as he pleases and has never had to think like a grown up about it - I know several woman like this, and it was completely fine, right up to the point they had DCs and suddenly were surprised these men had no idea what the financial implications of dropping to one wage would be, because they'd never had to worry about the figures.
He can see he has the money, that you might need it to be spent on something else makes sense to you because you are used to being aware what bills are there and what the family outgoings are - but you have allowed him to opt out of the side of being a grown up that involves budgetting for bills, not just for fun things. You've also been fine with him having lads holidays without you having girls trips away, so he's never had to think if this was selfish.
time ot sit down, not just talk about the trip away, but set the bugets for the next year with 2 DCs, get him to do it, ask him to look at all the numbers, what's going out, what's coming in, how you'll cope when you drop to SMP, then when you drop to nothing, or will you have to go back to work then. Get him to have all the passwords and do a spreadsheet. If you can only afford this by going back to work a month earlier, have you asked him if he thinks it's fair that he puts his short term desire to go away above the need of his about to be born DC to have an extra month with you at home. Make him also look at the costs of childcare, what your budgets will be when you go back to work, how much less you'll have coming in and roughly what the extra costs will be.
I would also knock on the head the idea he can never have a lads holiday again after you've had DCs, but this year and probably next year might be unaffordable, but after that, there's no reason why affordable trips can't be continued - it's a case of working out what's affordable, and he might have to push the other "lads" towards a cheaper trip.
But most importantly, stop mothering him, stop letting him opt out of the grown up conversations, then being surprised he doesn't act like a grown up when you want him to.