Yeah - it's a duplicate answer. RP's take on this question is here.
Good start, and no, I ain't gonna shout at you about cheating. You didn't know, it is 100% this jerk's fault.
Moving on though, is gonna be tricky. Look at yourself for a minute and tell yourself exactly how much rage and shame you feel. Don't bother telling me, I can guess. At the moment you are brooding, right? Little revenge fantasies, calling yourself 17 different kinds of stupid, all the usual good stuff we do when we get hurt and tricked into hurting others.
You fancy telling his wife, don't you? Letting her know what a snake he is.
Don't.
Not so much for her sake, and certainly not for his, but for the child's sake leave his wife some illusions. While I am a huge fan of beating the crap out of guys like that, your best bet is to, in the delightfully charming British phrase, "blank him."
He simply ceases to exist as far as you are concerned. You don't answer calls, delete any texts or e-mails unread and, should you see him, look right through him as if he isn't there. He is no more important to you than an empty burger wrapper.
Why waste time, energy and emotion on a complete wanker? The best revenge is living well.
Go do so.
Did you ever think the two things might be related? Your affair and his child? I'm having trouble being a part-time step mum (and I mean to the point of literally tearing my hair out) so it must be incredibly difficult for him being a full-time dad to someone whose needs are greater than many children's, who are a freaking handful in the first place.
I'm not excusing it, and I don't like it, but you were probably just some stress relief. I don't know, obviously, the guy could be filthy rich and have a million doctors and carers there 24/7 for the kid, but probably not. All that you will do by ripping him a new one is ruin his family. And maybe it should be ruined for his sake, but like MM, not for the child's. I'm pro-honesty, but you both know the affair happened. It's time to let him deal with it in whatever way he needs to.
Tough call, great answer.