I can't help but get turned on when my boyfriend smokes.
Why does that turn you on exactly?
I like to see him smoke. It feels great.
While you should be allowed to have specific kinks and turn-ons, one that hurts you significant other isn't one to have. You should never encourage someone to actively do something that is bad for them, and instead you should be caring about him instead and trying to prolong his life however you can.
But I like that he smokes.
Just because you find it a turn on, doesn't mean he should keep smoking and jeopardizing his health for it. How about weaning him off onto less harmful things, like vaping? You will still be able to see that turn on, but it's a little more better for him!