Do you think aliens in ufos have visited Earth? (And all things UFO)

He violated the terms of his NDA the moment he acknowledge the existence of the program he was "read" into, and that is speaking from personal experience. He is no longer a government employee and he says that he wants to "blow the whistle" on UFO's, then do it! Don't say I will answer your question, but only in a SCIF!
That's not really true though. Just acknowledging a program exists won't necessarily violate his NDA. What he can't do is speak to actual classified information. The existence of a program itself isn't classified. And to some degree, even information within said program may or may not be classified. He does have some protections under whistle-blower status, but that doesn't apply to classified information and documents.

He's not going to put his career at risk any more than it already is by speaking to classified information outside a closed session. He still has a lot to lose.