Honesty is the best policy, please trust me!
Last Easter my boyfriend did exactly the same as what you did - he went to a party, got really drunk, and made out with some girl. They touched/kissed and he assured me that he didn't sleep with her.
He told me on the same day that it happened because he felt so guilty (honestly, he nearly cried because we were both so upset!) but I forgave him instantly because I understood how hard it was to confess such a terrible thing, and he was so brave.
Obviously I still felt really hurt, but the fact that he was honest about it made me trust him again.
However, recently I found out that he wasn't being completely honest....
He told me before that the girl he cheated on me with was just some random girl, but a few months later he confessed that it was actually one of my friends.
Because basically all of my friends knew, and him, but nobody told me for months, I was miserable for ages because I felt like I couldn't trust anybody, not even my own friends because they KNEW!!
Now my so-called "friends" don't really talk to me anymore because the trust is broken, and everytime I think about it I get so upset. (Really, words cant convey how depressed I am about this...)
So, please don't make the same mistake he did, I beg of you. If you love him and you want your relationship to work, you NEED to tell him, because then it would be easier for both of you to trust each other again.
Also, I can speak from experience when I say that it's really, really crap to be kept in the dark when everyone you I cared about was keeping something from me. Please don't do the same to him.