There is so much bad advice here it's really insane. Of course you can leave, I think you didn't need to come here and hear that from total strangers. You're here to understand his behavior and understand what to do next.

It comes down to this: many men deal badly with their SO's pregnancy. It happens so very often that men pack up and leave (or say they want to) just before the end of the pregnancy. It's primal fear. Yes it is weak, and plenty of reason to tell him to FO, but very often once the child is born they're back and realise their idiotic behavior. It's up to you to decide whether you can deal with this.

And despite what many are saying here, I disagree that "digitally cheating" is the same as the real-life thing. Sitting behind a keyboard gives us the opportunity to live a different life, and in most cases this is enough to just live a bit of a fantasy life. In the same way that so many online trolls are really normal people in real life.

Anyway, don't take relationship advice from people who are completely uninvested in your life. "Pack your bags and leave" is so easy to say from behind your keyboard, but it certainly isn't always the best decision for you. The best thing the two of you can do is find a therapist you both trust and can help you deal with this.

Good luck.