Why would a guy act this way and when should I just give up?

Now I’m not a fan of the relationship advice on fluther deal, mostly because I usually feel that you need to know at least one person involved well enough to give advice specific to them. However, I am at a total loss now, and I thought I’d ask for some fluther insight so the situation could be looked at from a new set of eyes.

The story goes like this… I hooked up with a guy I worked with this summer, and after some rather complicated situations, we didn’t speak for a few months. I was home for Thanksgiving and received a message via facebook (mature, I know) asking me if I was in town. I didn’t get it until less than a half day before I left, but I responded saying just that. He sent another just before I got home for Christmas, saying he was sorry he missed me last time and asking if I’d like to hang out when I was home again. I wasn’t entirely sure that I wanted to, given our history, but I told him that he could call me if he liked. He didn’t, for about a week, so I sent a text message asking if he still wanted to see me. He didn’t respond to that, but two days later he called and left a message, asking me to call him back. I did, and we set a date, and ended up hooking up again. After all was said and done, he got up quickly and said he had to be up early in the morning. He drove me home and said it was good to see me, at which point I informed him that I’d be home for about a month. He said he’d call.

About a week later, he called again, and we planned to hang out after I got off work. I ended up not having to work, so I went over and we hooked up again. Once again, very shortly after we were finished, he came up with an excuse for why I couldn’t stay. He did walk me to my car and kiss me goodbye, however, which I found a little odd, seeing as he essentially kicked me out. Five days later, I decided to call him because I was bored. It rang twice and he picked up and promptly, without saying a word, hung up. I was pissed, and decided that I was done, I wouldn’t contact him again, assuming he wasn’t interested in talking to me again (though I have no idea why).

Wrong. Two days later, he calls me, while I am at work. He doesn’t leave a message. When I get off work, I return his call, but he doesn’t pick up. I don’t leave a message. The next day, right as I am getting to work, literally minutes after I clock in, he calls again. He doesn’t leave a message. Yet again, I return his call, but he, yet again, doesn’t pick up. I call again, a few hours later, but still no answer. That was yesterday. Today, he didn’t call, but I called twice, a few hours apart. I left a message only the second time I called, just saying that he should call me back. He still didn’t answer. I know he was at work (remember, we worked together, but now we work at different stores within the same company), because our district manager came into my store earlier tonight and said something about it. It’s now almost an hour since he got off of work, and no word from him at all.

What’s the deal? I’m incredibly confused, and I know this isn’t a relationship in the conventional sense (it’s basically just casual sex), but I have no idea why he would hang up on me and call back, but not call now. Any ideas would be greatly appreciated! (Sorry that the question is so long…)