Just found out my(F34) husband (M36) had a kid he abandoned

tldr: my husband had a baby in highschool, and literally pretended it never happened for almost 20 years
I am in absolute shock, and I don't know how to proceed. Due to the sensitive nature of the subject, I don't want to confide in my friends just yet. Names changed, obviously.
Background: I met my husband in college, but we didn't get together until a few years after I graduated. I've known him since I was nineteen, and we've been married for 8 years. We have two children together (M7 and M4) an I'm sixteen weeks pregnant with our third. I'm very close with his family (My FIL, MIL, and two SILs) and I always kind of idealized them. My own father was kinda absentee after he divorced my mom when I was very young. Even in the early days of our dating, I talked with my husband about how important it was to me, that my future kids father would be ACTIVE in their lives.
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Last night, I got a Facebook message from a woman I'll call Anne. It basically just said "Please tell (husband) to message (Joe Lastname), Joe would really like to get in touch." It included a phone number. I showed the message to my husband, and he FREAKED OUT. He's normally a super levelheaded guy, but he just LEFT. I mean, walked out of the house, got in the car, and drove off somewhere (?????) still in his pajamas. I called him maybe 20 times, and when I called his family and told them what happened they all seemed weird too. My MIL told me it was "probably nothing" and to give him some time, which was such an obvious lie I didn't know how to respond. One of his sisters texted me that she would talk to him, and that it was "time for him to tell me."
So obviously my first thought was an affair, and I start freaking out even more. He came home drunk (FROM WHERE???) and you better believe I was furious. About him leaving, the drunk driving, what I thought was an affair, all of it. I want to say that this is incredibly out of character for him. If you talked to me 24 hours ago, I could tell you I could count on my fingers the number of times I've seen him drunk, and we went to COLLEGE together. Definitely he's never done anything even close to this since we've had the kids. Anyways, I confronted him, and he broke down sobbing on the couch. He explained to me: when he was sixteen, he had a one-night stand with a classmate, and she got pregnant (!). He told her to have an abortion (!!) and when she said no, he said he didn't want to be involved at all (!!!). And he didn't. This kid is NINETEEN, so for about twenty years he just...pretended it didn't happen. Pretended he didn't have a son! No visitations, no child support, nothing. (He did not tell me all these details. His younger sister told me some more details, once I called her back).
He said things in this conversation that truly shocked me. I always thought he was a feminist, but he said the most horrible things about this girl he was in highschool with: that she was trying to trap him, that it was on her to "use protection or keep her legs closed" (!!!!) stuff like that. He said he didn't doubt that the kid was his, but that he doesn't have an obligation to do anything, that he shouldn't be "punished" for the rest of his life.
I just keep thinking about this young man (my sons brother!!!) growing up without a father. And for the record, we are a personally pro-life household (and by that I mean I would never have an abortion, but I don't think it's the government's job to make it illegal) so I was also pretty shocked to hear that he wanted this girl to abort the baby. I know that's kind of a small thing in all of this mess, but our eldest was an "oops" baby when I got pregnant shortly after we married and my mind keeps going back to how loving and supportive my husband was. I honestly wish he DID cheat, because I feel like we could have worked through that. But now I feel like I don't know him at all. And his family! They all knew and kept it from me! I keep thinking of new bad aspects of this, the longer I sit here. Quarantine with kids was hard enough. Being pregnant during a pandemic is hard enough. But what the hell am I supposed to do now? I;ve been thinking about messaging Anne back to tell her and Joe that he has siblings. I've been thinking about taking the boys and going to my mother's house. I've been thinking about a lot of things, and I just feel so alone.


Source.