I think, if you like this guy, you have to consider your feelings for him more. You say you're best friends with him but that doesn't mean you shouldn't go for something you want. If you like him and want to pursue a relationship then, by all means, go for it! After all, if you stayed "just friends," do you think you could suppress your feelings for him?

Why not try flirting with him more? He seems to be really interested in you and maybe isn't sure if you like him. Guys can be shy and unsure of themselves so give him some guidance.

As for this whole "bet" thing.. I wouldn't take it too seriously. Guys always do this sort of thing with one another and it doesn't really mean much. If he likes you enough, he'll definitely get together with you. A little "bet" or joke with his guy friend is not going to stop him from hanging out with an awesome girl that he likes a lot so don't hold back!