Is he interested? Why wont he ask me out?

I have a co-worker than I thought might be interested. He does all the typical signs. The staring, the joking, the flirting, even when I am around he is louder as if he trying to get my attention. When he sees me he almost always strikes up a conversation. Earlier today I was talking to another co-worker when he chimes in with a comment. When my co-worker responded he said "I got her back and she got mine." While a little while ago we were finally by ourselves. I attempted to strike up a conversation but he was barely talking to me. It was always like he wanted me to leave him alone. What is he deal?