Is my friend bi or just a jokester?

I don't know. This is sort of a tricky questions. Literally almost every guy at my school is like this - and they're all straight (except for maybe 6 or 7) - so I'm used to it. My brother (straight) acts like this with his friends too. So I don't know if my advice is any good, since I'm sort of desensitized to this. All I can say is don't be too quick to judge, and if your friend IS bi, please accept him. He's clearly nice and cares about you as a friend.