Something is on his mind
Men aren't all that good at talking about their emotions, especially compared to us ladies.
That's often why when something is worrying them they bottle it up and go a bit quiet.
It could be anything - work, their friends, their family. Whatever it is, it can make them a little less chatty and it might make you feel like they are ignoring you.
If you think that there is something playing on your boyfriend's mind, just ask them. You'll usually be able to work it out together and it means you can stop worrying about why he's so distant.