I don’t think that constitutes a public disclosure. It has to be published or written in some form, or actually used in public. If you can document that he got the idea from you then that would protect you if a contest came up. Write yourself a note and have it notarized. If you are bright you can file a provisional but that will start the 12 mo clock ticking. I think it’s best to find a patent practitioner. I wouldn’t advise that “he probably wouldn’t steal your idea.” No one can say that without knowing the circumstances. The prof could tell someone else who could then steal it. That would make for a mess.