Everyone knows the rumors - including Roberts. He does not want to place himself in the middle of what would happen if he read the question. Can't really blame him.
The proper place for Rand Paul to out the name himself is on the floor of the Senate during normal proceedings - or on Twitter, like he just did.
Actually he is not outing the whistleblower.
He is asking a questoin about certain people.
So clearly the refusal to ask the question means this dude is the whistleblower