Most commenters agreed OP wasn’t wrong for outing Alex’s dad on Facebook. It may have been petty, but what Josh was doing was so strange and he shouldn’t get away with it. Also, it’s plausible that she “innocently” mentioned Josh being Alex’s dad, and didn’t realize he had never mentioned it before. Because it would be weird that he didn’t, right? If she played it up that way, it would seem totally justified, too.
One person brought up a good point to counter the “petty” argument: “Honestly, it seems like a pretty low-stakes way to test her hypothesis,” they wrote. “If she had just up and asked, ‘Are you refusing to acknowledge your son?’ there’s a ton of room for backtracking and gaslighting.”
One commenter wrote that their theory is he was trying to keep the fact that he has a son off social media because he’s trying to date again. Even if that’s the case, the dude is going to have to come clean eventually if he gets serious about anyone, and it’s all but guaranteed she will think it’s pretty awful that he pretended not to have a kid.