Why does he get so vague when we text?

So there's this guy who I sort of like, well he told me he likes me and so I told him I kind of did to. And he texts me all the time and even when he's the one to text me first (which he usually is) he is so one wordy! What does it mean? All he says is "ya" and "OK" and it really annoys me. I always tell him that I always have to keep the conversation going and all he says is "sorry" I mean, oh my god! What's up with that? It feels like he doesn't want to text me and I'm annoying him. But it doesn't make sense is that he's the one to text me. So what is up?
Why does he get so vague when we text?
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