He didn't say something before because he was being dishonest with you.
If you had made it clear that you wanted more than one, and he very clearly kept his mouth shut at that point, then I'm afraid the logical conclusion is that he thought 'Oh I'd better not say I don't want a larger family, or she might leave.' Now that you have a child together and it's obviously much much harder to leave, he can come clean.
Which is utterly awful.
And which is why you call his bluff and say, well I wish you'd said something about this before we had our first child, as it is a dealbreaker for me. We will end up splitting I'm afraid, as I want more children.
You put the ball in his court. And he will either say, well, that's what I really want, NO MORE - and then you decide whether to follow through and split. Or, and this is perhaps more likely, he will agree to have another because he would choose a second child rather than a divorce. In other words, he'd prefer not to have another, and will try his best to manipulate you so that that happens - but if you push back hard enough, he'll back down.
I have seen the latter happen before, I don't think it's that unusual (and in a situation where the man keeps quiet until the first baby has arrived, like here, I have NO sympathy for him at all). In the couple I knew, the bottom line was that he ended up a not massively involved parent and so didn't actually feel any desire for a second and given the choice would have plumped for just not doing it all again. His wife felt extremely strongly about it and basically said, I couldn't live with that and stay with you, so we'd be over. He ended up deciding that he preferred having a second child to being divorced - quite simple really. One assumes that if he truly felt, viscerally, stonrgly - that he absolutely hated the thought of another child (which is his right) then they would have split. But, he actually didn't. He just chanced his arm to get the result he would have preferred, and was knocked back.