Guy problems

I have no idea what to do. Here's the story, my SO of 4 years now used to smoke weed. I told him my feelings about it and he quit for me. But since my brother started doing it, he started again. He used to respect my feelings about it and not do it. But now he lies to me about having it and doing it, but I can totally tell when he does it. And I found it in a backpack. I don't want to have this shit around my baby when he/she comes. I'm 11 weeks and I feel like he's doing this now because he thinks he can get away with stuff now that I'm pregnant and he thinks I won't leave him now. But he's got another thing coming if he doesn't quit his shit. I just needed to rant. Any thoughts are welcomed.