from a bisexual, these are the possibilities:
1. he is thankful that finally he has a close friend to hang out with who would understand his being half-gay. in my experience it is harder for a bisexual to find gays and straights to accept us. the gays would think we are all in denial and the straights would live in fear that if we would not snatch their girlfriends, we'd definitely molest them or something!
(sorry for using the words "snatch" and "girlfriend" in the same sentence in a gay forum… LOL)
i think he's just basically thankful to finally have someone to talk to about gay/bi stuff.
2. he could be just joking. i have more straight friends than gay/bi friends and they tend to joke a lot about these things when they are already comfortable with you.
majority of the times, they don't mean it seriously.
the open-minded straight guys even have a lot of questions about gay/bi people (albeit, most of those questions seem stupid to us)
3. he could be attracted to you and was lightly flirting with you or giving you enough hints to see if you like him too without sounding/appearing like a desperate man-slut!
these are just the possibilities that i can think of.
the wrong thing for you to do is to assume any one of these and act accordingly.
the right thing to do and the decision that will not damage your friendship would be to talk to him about it.