I feel like I am missing something, why are everyone in the manga berating the guy for what he said before she left? All he literally did was bring up the topic for further discussion (something they definitely need to do), and when she assumes he is going to say its an issue, he replies thats not what he was going to say and mention that its a lot of responsibility etc. (clearly implying that they need to discuss it and not thoughtlessly ignore the topic, and likely also time for digesting the news). Then she suddenly says he is saying thoughtless stuff about her child? Wat? When the hell did he do that? He hadn't even gotten to raise the topic yet? And yet everyone is calling him an idiot and saying he said mean things?
That said, if he had gotten to raise the topic, who knows that he was intending to say. From the park scene, my guess is he wasn't going to say he is thrilled by the idea of suddenly getting a 4 year old child thrown at him and his life out-of-the-blue (noone would, or perhaps I should say that if that is someones initial reaction I would start wondering if I can truly trust them with such a position). The fact that she intentionally kept it hidden (alongside lying about her job) for so long is sure to raise even more concerns than merely making it a surprise throwing him off-balance that would require him some time to consider and internalize (which even if that had been all, would still be understandable for someone to be less than thrilled). The fact that he never outright rejected the news despite all that, should serve to show he was still valuing her enough to decide the effort might be worth it and needs further thought and info and that he's actually possibly open to still progress the relationship (which he later did). Hell, he didn't even freak out whatsoever (he just found out she were intentionally conning him!), and yet directly went to the route of trying to establish a baseline from which he can decide his future actions, brushing the trust-issues aside for the moment.
Overall, all characters reaction to what little he said, almost makes it feel like the translation might be lacking, or at least some nuance of it.