How do I tell my boyfriend to stop smoking?

My boyfriend been doing these dab pens and smoking weed and does that Juul thing. I personally hate when he does it because of the health risks that come with it, and I told him that, but he just doesnt believe me. I'm trying to get him to quit for health reasons and he said he is trying to quit all of it, but he's doing it even more and doing it ever single day almost all day. How should I tell him I want him to stop? He already has a lot of health issues besides the smoking and I don't want anything else happening to him. I really want to help him quit so bad because it's so difficult to get an 18 year old teenage boy to quit something like this. Pleade help :(