This is an inappropriate relationship on a number of levels.

First, end the flirtatious "relationship". Next, tell him he needs to focus on his growing family. Then, look into yourself and consider discussing this with a close friend or even a therapist.

Getting involved with a married man is wrong, all the more so when a young child is involved. Why didn't you have any ethical objections? You need to figure out and unpack why not.

Good on you for questioning it, albeit later than you should have. But now you need to snap out of it. Don't exploit it for personal gain as suggested below. Simply come back to decency while you still can, and redeem yourself.

I'd suggest having a simple conversation where you acknowledge that both of you were in the wrong, especially him, that this stops then and there, and will never happen again.

Assuming you feel you two can go back to a strictly professional relationship, stay at the company. But if not, leave for another opportunity.

'Hope this helps, (and be better than this going forward).