Why hasn't he asked me to be his girlfriend?

I've been seeing this guy for about a month. We do everything a boyfriend and girlfriend would do, we see each other all the time, always calling each other, and we don't even dance or flirt with other people. I like him so much and he says he has strong feelings for me too, and really likes me. He's even making plans for MONTHS from now with me, talking about what we'll be doing in January. However, he still hasn't called me his "girlfriend" and it's pissing me off. A lot of girls out there still think he's single and are always flirting with him. Even if he doesn't respond, it makes me agitated. Do you think he hasn't asked me because he doesn't really like me? I feel so used, and really miserable.