Thank you for trusting your question to JA today. I am a licensed attorney with over a decade of law practice and over 20 years of experience in the legal field. I’m happy to be of assistance.
You would have a good claim of defamation of character. However, finding out who has been told can be very difficult, because he isn't going to volunteer that information. Perhaps, after a trial, it might be possible as part of the judgment for him to be required to list those told and to then notify them that the statements were not true.
But that is some time from now. You first have to file a suit, get through the discovery process during that suit, and most importantly, establish the financial damages from the defamation.
This is a dying lawsuit form, because it is so difficult and expensive to even try to establish the damages. Most people can't establish, with certainty, enough damages to make the cost of the trial worth while. This is why you might want to consider first just using a "cease and desist" letter from an attorney (a limited engagement, so the cost can be controlled). Such a letter would certainly threaten a suit, which would most likely get the person to stop making these false statements.
If you have any further questions, please let me know. I invite follow up questions, so use REPLY for those. If you have no further questions then good luck going forward and please do not forget to rate my service with a top-three rating so that I receive credit for working with you today. Also, feel free to request me in the future, if you have questions concerning a different matter.