My friend is coming over. How do i ask him to experiment?

We are goth guys, 15, and never talked about our sexuality though I can almost guarantee that he is at the very least, bi curious, and so am I. Sometime next week he is coming over to spend the night, should I ask him before he comes over, or sometime during, also how should I approach the question, like what topics should I bring up, if he says no it will be awkward so how should I diffuse that. It's not like he'll get offended and leave, he's not a homophobe or anything, in other words, (to all you who would write anti-gay things) he doesn't overcompensate for his own sexual insecurity. Also what do you guys think we should do? What would be going too far?