Sue him for what? Not giving a good review of you to the company who you interviewed with?

Take that energy and apply it somewhere else. There is absolutely no case against this guy and you will end up paying for his costs fighting what will be deemed a frivolous lawsuit. People are allowed an opinion. I would be surprised if any lawyer would take the case. The hard reality is that this exists and you must learn how to manage this contact. And the truth is you really can not be certain of why they turned you down. Do yourself a favor and let this angry idea go.

First, don't use him as a contact. Find someone that can reasonably attest to you working there. Another manager, pilot or friend. No airline will be of the wiser.

Good luck. You will find a decent flying job if you persevere.