The worst case is that you'll get a negative. Not the end of the world and you can always respond in a professional manner for future buyers to see.

Ebay won't do anything unless the buyer opens a case. So I would stop communicating as there's nothing you can do. If he messages you, you could just say something like "I'm sorry, but I am unable to assist any further". 

It's also entirely possible that he has raised a number of cases against sellers and is on Ebay's radar for abuse of the system. We did see that here recently where a buyer was trying to get the seller to refund and it turned out the buyer was ineligible for the MBG policy and could not open a case.