You had unprotected sex with him before you were together or had been tested? I suggest you DO get tested if you haven't even bothered to be at this point.
I agree with the others posters that this is why people define relationships. If you aren't comfortable sleeping with someone who are sleeping with someone else and giving them the same treatment they are giving you, you shouldn't be.
Sometimes we learn things the hard way. This will certainly be a lesson for you. People aren't going to make commitments that aren't required of them, they will make the most of the opportunities they get. Men want sexual variety more than women do so it is unlikely that the situation could have been the same the other way around. However you can imagine dating a guy you felt so-so about, and then being offered a date with your favorite celebrity, who turned out super charming and seductive - you wouldn't pass that up, especially if you didn't owe anyone anything.
Of course there is a question of decency. The decent thing to do would for him to be clear and open once he realized you were developing feelings - your birthday was certainly that point, I've never even heard of someone celebrating a birthday like that while not being a couple. I have never demanded a single thing on my birthday from men I was only dating. It sounds a bit as if you wer assuming exclusivity.
I don't know the details of what he told you that Saturday. From the sounds of it, he has never outright lied, just avoided the subject - which is the normal thing to do these days. "Lying by omission" (i.e. withholding facts knowing that a person would feel differently about you if they knew everything) is not breaking any laws, but still morally on the edge.
I can understand that you are bothered - it feels like his words, how he's spent his time and how he feels about you is a lie. He DOES like you a lot, otherwise he would not have agreed to be exclusive. We don't know how this 'exclusivity' happened though, as you haven't said who of you initiated the subject.
If you want to talk to him about it, do so. However be careful of how you bring it up, only mention the relevant things and be prepared that he may be furious of your snooping, to the point where he might not want to be with you anymore.