I think alchohol can completely change someone. Drawing from my own experiences with my brother, when we were kids we were best friends. I looked up to him, he was just, "cool." He fell in with the wrong crowd, then he started drinking. When I look at him, I remember my brother from back then, and see that he is most definitely a changed man. So yes, alchohol can change you. I do not pretend to know much about this subject though, I'm here for help just like you but, all I can say is that I feel for you. I hope your situation is resolved, but for now you just have to understand that alchoholism isn't really a choice. He's not choosing alchohol over you, he has a disease, something that takes away all of his self control.
Really, all I can say that having a person you are close to become someone is confusing, frustrating, and simply hurts. Try to remember the good times you had with him and that he is sick, it's not your fault, and from the sounds of it, it's not who he really is, but alchohol changes you like that.