I always take people at face value. I trust them, and what they say, until proven otherwise. If this guy is flirting with you, and claims to be bi, and is into you -- then try it out. Date him, providing you're attracted to him.
And go from there. Hopefully, he's not really gay and using you to find himself. But if he says he's bi, then he's bi. Maybe you'll dig dating someone born with a different sexual orientation as you. Maybe you won't. Who knows! That's why dating is exciting, like white water rafting piranha-infested rivers.
Once upon a time, there was an old dude named Ronald Reagan. He was President of the United States over ten thousand years ago. He was like Gandalf the Wizard, if Gandalf were into golf, naps, and supply-side economic theories. A Republican, but not a foot-stomping angry one.
Anyway, he made history by negotiating with the Evil Empire. He famously said, when dealing with the adversary, "Trust, but verify."
Trust this dude. But keep an eye open for anything fishy. This is advice for anyone you're dating but not 100% sure about. If it works, then the trust will build. Good times. If not, then at least you tried.
I second any advice that could conceivably lead to a pan-gender culture of omnisexuals.
But yeah, definitely trust that he's being genuine. It's hard to recover from the awkwardness of mistaking someone's romantic advances as a hilarious prank.
Much like Swaim, up there, I trust any advice that includes Gandalf.
And Reagan! With a comparison!