Youre fine. Tell him to feel free to bring a lawsuit but he is open to reviews as part of posting on Zillow. In addition, he needs to be able to prove in a court of law that what you said was false. On top of that, tell him that he needs to understand that if he does sue, you will counter sue for damages, court/lawyer costs, defamation of character for questioning your review, time off work, mental anguish, and partial costs of the home you lost out on for having to deal with his BS.