My boyfriend occasionally drinks?

alright ill try to make this kinda short.. alright this probably sounds pathetic, idk. alright so ive been dating this guy for ohh about a month and a half not very long i know. we see eachother almost everyday sometimes a little less. anyhow yes hes of age to drink and he doesn't even get to carried away with it.. tonight is his second time even going out to drink. he promised me that it would only b one drink and i do believe him because im trying to trust him. he hasnt showed me any reason to not. last time he went out he got drunk .. he puked it out hey he was celebrating.. but for some reason this issue bothers me badly. like i feel as if im going to cry. im thinking its cause i have a mother whose been an alcoholic since i can remember an older sister who drinks and gets carried away and my father use to but not to often. i told this to him and i also told him i need to deal with this. tonight in the car with him he was kinda stern asking me if this was going to be a problem everytime he wanted to drink.. he told me yeah he does like once a month.. i just dont want to be that girlfriend that locks her boyfriend away.. i know im the problem. i just need hellp dealing with this..any suggestions:?