Most potential cover-ups are relating to seriously confidential matters. That is how silence, while children suffer, happens. This may or may not be a cover-up.
That is why a full investigation of seriously confidential matters brought into the daylight must happen.
No one should fear an investigation. No one should oppose an investigation.
If he did not want to give information while on the plane he could have done so in the days that have followed.
He needs to let his yes be a yes or his no be a no. He needs to answer.
But attacking a potential whistleblower as violating confidentiality...that is just not going to work.