I'd say the bigger question is why he feels that he can't say change of plan he's off to the pub tonight. And I think that the reason is fairly obvious op - because of how you react and your past responses.
As you have said, he pulls his weight with the children. You have obviously opted to have a second when you already have a young child, yet you are relying on him to help after a full day, even when you knew it would be after 8.
Yes, you're pregnant but that shouldn't mean that he has to run to heel! How are you going to manage with 2 children?
Yes, it would have been nice for him to make plans and told you in advance, but tbh I imagine he didn't want the earache before, knowing he'd be getting it during and after from you! And I understand that from the comments here.
Now if they're clients, that's par for the course. You don't mind the paycheck!
By all means be a prima Donna and go out with no notice Friday. All that really shows is you missing the point that YOU and your reaction are why he doesn't inform you. Most relationships should be without this stress to manage a one off change!