I do not think that he is in any position to judge what is the worst thing you can do to him. In other words, his fear that you may call his brother may cause things to change in such a way that he does not want. Alcoholics just love their secrets. I think you should probably call his brother to let him know what is going on. Perhaps he can offer you a point of stability in a very unstable situation. Maybe he can let you stay with him so you can clear your mind a little. And at this point, I would not plan for the wedding to happen on Wednesday either. That would be a very bad thing. If he seeks help for his sickness, then there are many possibilities for the future, but that help will need to come first if he and you are to have a future together at all.
I am so sorry to hear about your situation. Please take care of yourself first and worry about him second.