Seems to me the major issues are if you really "know" he is gay and if so is there anything about your relationship that might preclude or be complicated by adding sex (such as co-workers, boss-underling, etc.). If you are okay with these what is the worst he can say/do, if he is genuinely a friend with the positive concerns for you that implies (at least for me)? Best of luck!!!

Yet more suggestions for breaking the ice might be to ask what he likes best about being bi, what kind of sex he prefers, what kind of partner he likes, and such focusing on him. It could be that you are a great friend but do not fit into a particular like or fetish of his. Or, you might ask if he's ever fantasized about you? Thinking more about your situation, "experimenting" is sounding like fun!

Let us know how it turns out, one way or the other!!!

I met a guy once in the laundry room of my apartment. We chatted and he seemed nice and I thought he was bi or gay so I asked him if he wanted to come over for a beer. He said yes and after about four beers there was a lull in the conversation and I asked him if he was circumcised. He said yes and I casually asked if I could see it. Out it came. Eight inches of beautiful cock.
I just started sucking it and we had a couple of great days together. You can read the story on my blog. "My friend Darrell".
If you ask if he's circumcised it's just a yes or no answer and you can go from there.

ask him

Ask. Only way to find out. I wish you luck

Just tell him you want to suck his cock. If he doesn't run away ask him if he want's to now.