What should I do?

Ask him straight out. There are couple of problems here. He can't be too bright if he's lied to you and told your girlfriend the truth, doesn't he know she'd go straight to you and tell you what's up? What man wouldn't know that? Is he playing games with you? Was he trying to act 'cool' by saying he 'made out' with some girl? That would be just childish. He did originally confess to you that he was with that other girl, it seemed genuine that he wanted to be honest with you. Maybe he needed that day with her to show him that you were the girl he wanted. Obviously I can't speak for him.
You need to start a dialogue with him about this. You need to know where you stand in this relationship, girlfriend or just friends? and he needs to know that this behavior is unacceptable if he wants to continue seeing you. Whatever happens, the very last thing you need in a relationship is someone lying to you. Relationships are built on trust, whether they're friendly or romantic.

Well, you said yourself that there's really nothing official between you. It sounds like he might feel things are headed in that direction (making something official) otherwise, why would he have any remorse?

Because nothing is set in stone I don't see how you have any right to really yell at him, even though it probably hurt your feelings, I don't think you'd be in the wrong to let him know your feelings were hurt a little because of it, but don't start a fight over it.

Maybe you should ask him where he thinks your relationship is going, maybe it's time to make it 'official'. If you do make your being together official, I think it'd be ok to let him know that sort of behavior (drinking with, and flirtily holding hands and hugging on other women) would be unacceptable going forward. In fact, he probably knows it, or he wouldn't have brought it up.
Best of luck.

You've been hearing it all of your life.

Beware of the drunken player in the room

Never let yourself become the "fallback girl"

and

Be careful of the people who are Dr. Jekyll when sober, and Mr. Hyde when drunk

Because of your previous conversations, dates, and basically the building stones of a good relationship, you actually DO have the authority to call him out on his recent jackass-eries. I believe that the reason that he confessed these things to you is because holding hands and hugging are forms of PDA, however they are not the same as Kissing and the escalation of kissing.He confessed because he knows that he screwed up, did something that might start rumors, and decided that he respects you and wants this relationship enough that he is willing to tell you when he's embarrassed himself.

You're right, when you feel like you shouldn't let him off of the hook so easily. In fact, part of the reason why we confess moments like this one to people that we respect, is because we are looking for a fair punishment, or a good long discussion in order to solve the problem, or to feel like we got what we deserved and can therefore move onward. Lay into him a little, make him tell all of the details to you in person, talk about what when wrong that night. Was it the alcohol? Was it the people around him? Is he easily distracted?

Find these things out in order to
1. Better know each other in situations such as this "party situation"
2. Let him know that he is answerable to you when he's screwed something up
3. Be able to keep each other accountable