Hi, I'd like to help you with your question.
It sounds like your boyfriend may have a problem with drinking. I say that because one, you are concerned enough to bring it up to him. Two, his friends and/or family are bringing it up to him ("I get defensive when people tell me I drink too much"), and third, because of the evidence in his refrigerator.
Oftentimes, when someone uses alcohol to the point of abuse, they become defensive when someone points out their use. This is called denial and it sounds like where your boyfriend is right now. Denial is the first stage of a drinking problem. It is also the hardest to break through. If the person breaks through their denial, they are well on their way to accepting their problem and their chances of becoming sober increase.
There are some things you can do to help. One is to contact Al Non at http://www.al-anon.alateen.org/. They can help you find ways to help your boyfriend. They will also provide support for you and resources to help you both.
You may want to try an intervention. This is where you and some of your boyfriend's friends and family get together and confront him about his drinking. This is best done with an experienced Drug and Alcohol counselor helping facilitate the intervention. The counselor can help guide the meeting and make sure your boyfriend doesn't become too defensive and walks out of the meeting.
You can also seek counseling for yourself. This will help you cope and give you a chance to express your feelings about what is going on with your relationship.
Your boyfriend may also have trauma associated with his childhood. What he experienced sounds like neglect. Given that he experienced neglect on top of his feelings about being adopted, he could have a lot of unresolved feelings he needs to deal with. You can suggest he come to counseling with you based on just working on his childhood abuse and see if he is willing to try. Let him know you are concerned and you want to help. He may reject the idea of counseling, but it will open the door and possibly get him to think about it.
Here are some books that may help you. One is called Love First: A Family's Guide to Intervention by Jeff Jay, Debra Jay, and George McGovern. Another is When Enough is Enough: A Comprehensive Guide to Successful Intervention by Candy Finnigan and Sean Finnigan. You can find these on Amazon.com or your local library may have them available.
I hope this has helped you,