The problem is there are simply too many unknowns and you are already nervous about the situation.
Baby may be born a little early and you may feel absolutely fine immediately after the birth and skipping to the swings/ park with both of them on day 3.
Or
There is the possibility baby will be a couple of weeks late, you might need a c-section etc etc….
Ultimately, the actual crux is that you are feeling vulnerable and your DP is the cause of it.
It’s not an ideal situation…
This is a huge ‘life event’ that for most of us will only occur a couple or so times and will always be memorable- hopefully for all the right reasons.
If you have to say you don’t want him to go - he will see himself as the injured party who is being ‘controlled’… Not the best memory
If he does go, you may feel vulnerable and quite possibly (enduring) resentment that he isn’t putting your family first during this time…. Not the best memory.
An honest conversation about the unpredictable nature of your family’s immediately future and events and how vulnerable you are feeling would be a good start.
Does he realise how you are feeling? It may be that you normally deal with everything so matter of factly that he doesn’t know how you truly feel???
If he does appreciate how you feel & still wants to book a four day jolly abroad for a ‘friendship group’ friend - that seems to be a bigger issue altogether….
I hope you come to a happy ‘compromise’ that you are comfortable with.