Whistleblowing is only whistleblowing if you actually whistleblow.

Threatening to whistleblow is not whistleblowing at all. It is threatening.

The only allowable case is "I will whistleblow you for X unless you stop doing X".

For any other case of "I will whistleblow you for X unless you Y", that is extortion and that is a serious crime in many jurisdictions. For instance, "I will whistleblow you for X unless you give me $100", that is plain extortion.

Now, in this case, you haven't clearly stated what exactly he wants from you. Has he already made a quid pro quo demand like a favor, lunch, transfer of workload, money, etc.? Or is that the next shoe to drop?

He's already placed his head in a noose. It's normal for two different people to recall conversations differently. If you inferred from his conversation a quid pro quo, all the evidence weighs against him. His denial is not credible, and a search of his desk will reveal copious notes as to your activities, proving your claim.

The point is, this is a very petty and foolish man. Very sad for him.

The kindest thng might seem to be to ignore it. But to protect yourself, this does seem like a "race to HR" situation. The first one there gets initiative: the second will be on their heels in defensive mode. I would think about what he has really been asking for, then go to HR and tell them what he's been up to.