"Probably not." It's not a definite you're not, but that's just because Bigby is taking into account the fact that this could just be some mundy kid with an overinflated sense of ego.

But it doesn't seem like it.

Though that leaves the question of what Five is, if he isn't just some mundy kid. He can't be a Fable, surely Bigby would've known of him already then.. but what does that leave?

"You don't seem like one to me. Or are you saying I'm wrong about that?"