What do you do when your boyfriend lies to you?

Hey my boyfriend and i have been going out for over two years now and we have this thing where we are against smoking and getting high with out friends. i had told him i would break up with him if he did smoke or get high. He did it in the first month of our relationship but i let it go. we never fought but for the past 2 months we have been fighting non stop. Recently i found out from a friend he has been smoking every night as a stress reliever and i confronted him. He doesn't even seem like he cares that im mad and he wont talk to me about it acting like i did something wrong. he also made me look dumb because all of our friends knew and i was never aware. i also asked him if he would keep doing it and he answered in text "idk" and thats all he ever says. We are always together but when we leave each other he goes and smokes. so its very hard to cope with and i really just don't know what to do.