Why won't he talk to me??

O.k me and my boyfriend have been together for 3 1/2 years. He has always had a hard time opening up and communicating with me. If I ask him a simple question he will say 1 of 4 reponses: no, yes, i don't know or i guess. I'm not looking for a 1 word answer answer i'm usually looking for an explanation. Example: I'm trying to read his pay stub. I said baby where does it say about how much they took for the benefits? He replied its there. I said I don't see it. So I asked how much did they take out? He replied I DONT KNOW. If the shoe was on the other foot I would have got up and showed him how to read the pay stub. He is not a conversation starter more like an ender. He tries to be polite and and ask how my day was at work but he doest show any signs of interest when I proceed to tell him. He won't question or even respond. If I ask him how his day was he'll respond: long, boring or good. End of conversation!! Its driving me nuts. How do I get him to be more open w/ me?