He's a total creep, sorry OP.
You're having problems = he starts sniffing round an ex.
You're pregnant = he starts sleazing around a woman young enough to be his daughter.
And he's her boss.
Poor woman - I've seen a few threads previously written by worried younger women in this position, nervously trying to tiptoe through the harrassment. She's probably thinking fuck, I do not need this from my boss.
But in answer. No, this IS him, and he's not going to change, is he?
He's a nasty cheaty sleaze. If he's not cheating on you now, he will be at some point. It's a given.
You would be smart to leave now, while you're young enough to start again.
Honestly, don't stay with this creep for another ten years before the final inevitable split after the actual affair.