He doubts if he said yes that he'd get an accurate answer out of Bigby regardless. Or maybe he would. Maybe Bigby would tell Jason just what he gets out of this; if it's some sense of superiority, or if he's doing it to make up for some foul thing he did in his past, or if it's more along the lines of why Jason does it: because people are idiots and he's not going to let them be idiotic enough to just keel over, that'd be stupid.

"No." is the answer he decides to give, leaning a little more heavily on Bigby because the guy seems like he can handle it, and Jason's not carrying himself if he doesn't have to. Bigby made his choice, he can suffer the consequences.