My boyfriend goes to hookah bars with his friends and I am very adverse to smoking. What should I do?

I love my boyfriend and I'm not going to leave him just for this (so don't even suggest it), but him going to hookah bars with his friends makes me uncomfortable. Most of the deaths in my family have come from smoking related cancers, so the idea of him doing a similar thing they did concerns me. He doesn't go too frequently, fortunately, but he used to smoke before meeting me and I don't want him getting back into it. I've already told him how I feel about smoking and he's said he has no intention of going back, but still...
What should I do? I don't want to put him on a leash, or make him feel like I am, so do I tell him how it bothers me that he does hookah or just let it be?
My boyfriend goes to hookah bars with his friends and I am very adverse to smoking. What should I do?
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