Under FISA he is not allowed to use wistleblowing as a defense.
Actually, it's worse than that. Two of the counts he's charged with are violations of the Espionage Act, which was intended to prevent US citizens from colluding with US enemies during World War I. Unfortunately, the law provides no room for affirmative defenses at all: if secrets were leaked, you're guilty, and the court isn't allowed to consider even the slightest sliver of the surrounding context. Did you uncover something illegal? Doesn't matter. Is this course of action the only one that would have turned up malfeasance by intelligence agencies? That can't be discussed.
The reason the Obama administration's insistence that Snowden come back to the US to "face a fair trial" is so flagrantly disingenuous is that the act that he's charged under, by virtue of its complete lack of defenses, is explicitly and intentionally designed to result in anything but a fair trial. They're inviting him home for a railroading, and it doesn't matter whether it's done in private or public: he's fucked.
You should watch citizenfour, which spends quite a bit of time on this specific issue of how inappropriate the Espionage Act is for Snowden's actions, and just how unfair is is designed to be.