I am you, only I broke up with my Alcoholic 5 months ago. We were in a serious relationship as well and just like your boyfriend, mine drank heavily only on the weekends. Never touched it during the week because of work. They call that "binge drinking" like StarCat said. Binge drinking is a form of alcoholism just that binge drinkers haven't progressed to the point of being every day drinkers YET. I tried to get him to stop. I begged, cried, screamed and yelled. I even offered to go to AA with him. NOTHING I said or did mattered. He drank and passed out every weekend. And it won't matter to your boyfriend either. He could continue to drink for the next 20 or 30 years like this. Everyone is right. He won't stop until HE sees a reason to stop. You can't make him stop, he won't stop, the only thing you can change is your involvement with him. He has an addiction and he will continue to feed that addiction whether you get in his way or not. Its not that he doesn't love you, its just his addiction is more powerful than any love he could feel in the world. I decided my life was too precious to wait for someone to give up a stupid drink and I got out. Take care of yourself first.
P.S. My first post on SR was also "Is my boyfriend an Alcoholic"