Ouch! I feel bad for the guy, and kudos for the OP for wanting to stick with him!

You didn't mention what his reaction was though. Some guys may not care - after the fact; I'm such a cynic if I found out now, that out about my ex (and for all I know, maybe she did cheat on me, then again, maybe she did not...don't know and wouldn't care now), I'd just go "eh!", then get back to whatever I was doing. Still, I assume he at the very least reacted badly, or you wouldn't be here asking.

Thing to keep in mind is he's probably going to have major trust issues now. As many guys do, he may project his experience a generalization about all women. And, MM pointed out, he could become jealous over even the most casual contact you have with another man.

How you proceed depends entirely how you feel towards him and the stage of the relationship. Again, since you are here asking, I proceed on the basis you care very much about the man and you're generally happy with the relationship and want to continue it.

Take cues from how he's been reacting. If he's been prone to frequent emotional outbursts since learning this news, that's actually good since he's confronting and dealing with his betrayal and not keeping it in, just try not to take anything he says personally - because he likely doesn't mean it personally anyway. While some misogynistic feelings may express themselves, again normal, I would worry if he's constantly and bitterly hateful towards women in general. Try not get defensive, just be a good listener. Give him some time, and maybe space, but make very clear you are there for him. Finally, since he could be come jealous at the drop of the hat, try not to put yourself in a situation that might trigger it. Vigorous physical workouts do wonders for relieving stress and anger.

He'll probably overcome this, as many people do. You can help by confirming what he knows deep inside under his hurt, his anger; there are still good people in this world. But, be prepared for the possibility that this has damaged his trust in women for a long, long time. God, I really hope this will not be the case, but just be prepared if it does come to that.

This could have been written by me if you changed 2.5 to 3 years and added a lot of other shitty behavior along with cheating with multiple guys. But my boyfriend knew about it before we started dating; it's was actually why he broke up with her.

Trust is going to be the big issue here. As MM and Mr. X said, he's going to have some trust issues for awhile. He was so incredibly betrayed by someone he was supposed to be able to trust more than anyone else, it's normal that he should have problems trusting someone in that position again. In order for him to ever do that, you're going to have to help him sometimes, humor him sometimes, and just bear with him sometimes. Don't take it personally if he doesn't trust you or is ever suspicious. Do whatever you can to reassure him if you need to and whatever you can to keep him from getting jealous in the first place. Be ready to explain "Yes, I did meet my old friend Bob for coffee. Don't worry, he's gay." or something like that.

Having dated a man who was wonderful in every way but totally emotionally broken from his wife (6 year relationship, 1 year marriage) cheating with one of his groomsmen, I can tell you that sometimes understanding is just not enough. Make sure he is upfront with you about his emotional availability. If he is ready, REALLY ready for an honest and lasting relationship, he will have issues that come up. If he glosses over everything, he's still emotionally tied up in and/or broken from what happened before you. Which is totally understandable (not trying to take anyone to task for being hurt over a toxic relationship), but you can't control falling in love with someone who can't return those feelings if you don't know that they can't return them. I'm still heartbroken about losing a man I loved and trusted because he wasn't ready to move on from his ex-wife's betrayal.

I was that guy, once. 10 year relationship, at least 5 other men that I know about (one was a groomsman, to LT's comment above)

It's hard... My wife and I talked about "triggers" - you know, the things my ex-wife did that would trigger that gut reaction in me (Buddhist would call it Shenpa). She had her own issues as well, so we both worked through it. I would let her know where I was, whom I was with and all that stuff (especially as things got more serious).

It comes to a point, though, that you have (that I had to) open myself up fully and accept that I am in love with is NOT my ex-wife. She is a totally different person and her actions will speak loudly. The man in the OP's question will have to do the same -realize the woman he is with now is not her and each person is different.