I finally did it, 3 months after my mom. I knew it would be ok with him, as him and I have similar views when it comes to religion, marriage equality, and many other social issues. I started a conversation about the Chick-Fil-A about to open near us, and how I don't plan on eating there due to their CEO's anti-marriage equality views. He agreed that its ridiculous how so many religious people hate gays and how it's unfortunate that gay people tend to have to "hide" because of certain people in our society. I then went on to say that I was bisexual. He said it's good that I was becoming comfortable with that and was supportive. I then found out a couple of interesting things. He had never known anyone who was bi, though he had known gay, lesbian, and trans people at one point or another. I'm glad to be the one who rounded out the "LGBT" acronym for him. Also, he said he now knew two LGBT people in his family. He of course wouldn't say who it is and I didn't want to ask. But I have company I guess.
Glad that went well! I found that it felt so good to get the facts out in the open with each family member - so good luck for the future.