>>6 He's probably looking for assurance that you're not uncomfortable about it. If you don't make it clear that you're totally okay and accepting of his sexuality, he's gonna be afraid to be open with you and will think he has to censor himself about his sexuality. Friends don't just come out after their friend say they're gay and go, "Well, I don't really like hearing about it," so they assume that's what your awkward silence means.
The next time it comes up, I would suggest continuing the conversation in that direction :o Be casual about it. Maybe ask when he first noticed he was gay or what it's like? Be curious but not stiff about it. Joke about some hot guys in the school? Or if he's not taking it well, you could just bluntly apologize about being awkward before (tell him what you told us - that you were afraid you'd hurt his feelings if you said the wrong thing and froze up).
I went through this a similar phase with one of my friends, and I'm not even gay. Unbeknownst to me, he was very uncomfortable about discussing homosexuality, and whenever the topic came up about including it one of our stories, the air just kind of went stagnant around us. After years, I finally managed to drag it out of him that he's just didn't like talking about it. Your friend may be afraid of being in my shoes - accidentally being too pushy about a sensitive topic.