My boyfriend might smoke what am i supposed to do?

okay well so far me and my boyfriend have been going out for four days. its been great i like him alot but today me n my 2 best friends were n my pool n we were talkin bout random stuff n i told them that he was bugging me and asking me if i would kiss him well then they started whispering n i was laughing about it coz i thought they were making fun of me for not kissing him well turns out thats not what they were talking bout they said that they(my b/f n my two best friends and my b/f"s big bro) were walking down to my house the other night to hang out n my boyfriends brother pulled out a cigarette n gave it to him n he smoked it(hm meaning my b/f) i had no idea that he smoked n it bothers me alot because half of my family is dead not necessarily because of smokin cigarettes but drugs n i dont want him to end up that way i want to talk to him bout it but what do i say if i do i really like him and all somebody please tell me what i should do i know it probably wont mess up our relationship but what if it does i want him to quit really bad coz if he doesnt i dont think i can really be with him im probably thinkin bout it way to much but it bothers me ALOT!! i hope somebody can help me