What is this guy doing?!?

I recently told a friend of mine that I liked him while we were at a party. He told me he didn't believe me at first and promptly got rather drunk before coming onto a friend of mine. When I asked him about it he said that he had no idea that I felt like I did and added "The whole time I was with her, I felt like I was cheating on you" before asking me to a party with the phrase "The I'll have a date." He's been hurt before and keeps his emotions very private. When we get together alone he's upbeat and seemingly relaxed and we talk for hours! Now this just gets odd. The guy has since formally invited my friend and myself to the party that was initially themed "lingerie". He told me that it was a silly idea of his flatmate and not to bother, just to come in ordinary fancy dress. My friend saw him later this weekend (!?!) and said that he had suggested she turn up in undies. He was being oddly flirty with her this weekend. Why would he do this? One minute it seems as if we're really close and that he really likes me, the next he's quite confusing.