Originally Posted by Arcanum
Remember that a friend can support you and not be gay/bi themselves. As a matter of fact, it's actually good to have a friend(s) that are heterosexual... just to get the perception of the other side of the fence if you get my drift.
If you feel he doesn't believe you then don't go out of your way to prove yourself, just be yourself around him and he'll eventually figure out that you weren't joking.
Just remember that just because you may be questioning your sexuality doesn't mean you have to experiment with the same sex straight away!