Boyfriend going to prom with another girl?

Me & my boyfriend have known each other & "talked" for almost a year, & we've been official for 4 months. In January, I asked him to go to my prom with me. Of course he said yes. It's March now, & uhmm...he hasn't mentioned his prom. My friend asked him who he was going to his with, and he's going with some girl that he knows. She graduated already, so it's not like this is her her senior prom or anything. She even went to a whole 'nother school than him. I haven't said anything to him yet. I'm thinkin, OK maybe they established that they were going together before me & him got together. Still, I think that if he loves me like he says he does, I should be the one he takes to his prom, not some other chick. I'm not sure, but I don't think either of them have gotten anything, like a tux or dress. If that in fact is the case, I don't see any reason why he's still taking this other girl. I'm really not OK with this. I actually kinda wanna break up with him cause of it. Then again I don't, cause I love him. Also, if I do break up with him, I would probably have to find somebody else to go to my prom with. & I really wanna share that night with him. What should I do?