DH is a liar

My DH keeps lying to me and I don't know what to do. He will lie about small things like smoking a cigarette, lying about what time he went to bed when he was out with friends (I had to work.) They're small lies but they're adding up!! I asked him this morning if he had smoked this past weekend, he told me no a few days ago and no when I asked him again this morning then he tells me yeah I did. I ask why he lies, he says because he's stupid. I can't trust him and don't believe him. I'm 32 weeks pregnant and am so confused