Help trusting my boyfriend

My boyfriend and I have had our issues with trust. When we first started dating, I told him I wasn't able to date someone who was smoking weed (something he did every day), and he promised me he wasn't going to do it anymore because he wanted to date me. At this point, it was the very beginning and it was me choosing what I could have in my life, not me trying to control him. I have previous bad history with people in my life who smoke weed so I have reasons for why I can't have it in my life, I don't think it's bad necessarily. Fast forward a few months and I find out he's been doing it again. He apologizes and then we go a couple more months and same thing again. At this point in our relationship, it feels more like I'm controlling him than me choosing the type of man I want to be with since he kept it up 8 months into the relationship, even though this isn't what I wanted it to be! This happened a couple more times and I finally told him I can't be with him anymore and we have a very emotional talk a couple weeks ago and he finally says he hasn't allowed himself to give it up in his mind but he's going to do it and actually try this time, something he hasn't done before. My problem is that he's going on a road trip vacation with his best friend who smokes a lot. Things have been better than ever with us lately, but how can I stop the anxiety and worry that he'll do it again and ruin our relationship on this trip? I don't want to bomb the relationship myself by frustrating him with my constant worry.