How do i convince my boyfriend to stop smoking weed?

okay so HES NOT A BAD PERSON! and im not just saying that because hes my boyfriend. but he knows how much i hate smokers and he told me he stoped and he willl but i keep finding out that he is . im not really as mad at the fact hes smoking but that fact that he keeps lying. but anyway i dont want to be one of those controlling girl friends that get all bitchy about it because i love him and i know i have no controll over what he does with his life but he has REALLLY bad asthma and i dont want it to get worse and i dont want to lose him so pleassseeeee help me convince him to stop. thankkyou sooo much