I would try to open the conversation with something neutral, like start a conversation on the topic of being gay, or someone famous that is gay or something just to feel him out.

Also, I wanted to add as someone who has ALWAYS been completely bisexual - this is a possibility too!

But you know too is the possibility that his acting out previously could have stemmed from him trying to deny the fact that his sexuality is other than that which he was projecting.

Just be delicate, let him know you are still there for him no matter what as a friend.