How do you "come out" to your homophobic father?

My brother is openly gay, I am asking this question on behalf of his boyfriend. He has told his mother that he is gay, and his mother knows that he is dating. However, he has not told his father because he is quite homophobic and his mother “doesn’t want him to”. He’s afraid he’ll disown him. I believe they are quite religious, he comes from a Greek orthodox family. He has come to me for advice on how to come out to his dad, but I don’t know what to tell him. I’m not a parent, nor am I gay. My advice is to just come right out and tell him, but he is afraid. Any other suggestions???