Responding to a Gay Christian in the Family

What should we say to our college-age son who claims to be a "gay Christian?" He believes God has told him that there's nothing wrong with homosexuality and that interpretations of the Bible to the contrary are in error. Worst of all, he's been sharing these ideas with his brother (who, by the way, is disgusted with him), and he wants to explain them to his younger siblings as well. At some moments I'm so angry I could scream. Other times I just sit and cry. We love our son, but we don't want this kind of influence in our home. What should we do?