How can I encourage boyfriend to stop smoking without nagging?

My boyfriend and I have been dating for 5 years, starting when we were 15 in high school. When we met he was a smoker (I know he was young, both his parents smoked). Apparently he told my parents he liked me and wanted to date me when I wasn’t there and my mom told him I wouldn’t date someone who smoked (which is true). That day he quit smoking and didn’t smoke for almost 2 years. Then when he was 17 he started smoking again but tried to hide it, but I could smell it on him and he ended up admitting to it.

Because we were at that time in a 2 year relationship and I loved him I didn’t give him any ultimatums. I told him I didn’t like it but that he was free to do what he wanted. But the truth is, it drives me crazy. He’s now more addicted than he was before and is literally a chain smoker. I’m worried about his health and mine being around it, it stinks, any time we go somewhere he has to have multiple cigarette breaks which gets inconvenient. My parents always comment how I smell like an ashtray when I come home from hanging out with him. I want to encourage him to stop without being controlling or nagging but I’m not sure how. He always says he enjoys smoking and will never stop, and living at home with 2 smoking parents makes it even more hard not to. I know in order to stop he will have to want to himself. Does anyone have any tips on how to go about this in the right way?