Well at least tell him that he did not "turn" his brother into a gay person. You do not need therapy to alleviate that part of his guild, right? Give him information to the extent that gay people are not "turned" gay, and, in addition, that there is nothing wrong with his brother being gay - it is not a bad thing to be gay. It is just a minority sexual orientation.

I am not saying that you will solve the entire problem because you won't, but at least you will have taken a baby step in the right direction.