you can't - he has to want to do it himself. If he does want to you can help by providing a supportive and non judgemental environment. How would you like it if he said he wasn't going to kiss you till you lose weight? Let him know the reasons why you want him to stop, help him explore ways of doing this if he wants to stop and let him know you are there for him if he needs your support. And don't take it personally if he slips up. If he wants to he will get there in the end.

I found getting pregnant stopped my man from smoking!

Just tell him you are no longer prepared to kiss an ashtray. Worked for me!