I believe he would not have a case. If he only orally dictated a synopsis to you he would not own any intellectual property. Copyright
rights only come into existence when you fix a expression in some tangible form such as a writing, a video, a photograph, painting, etc. Oral expressions are not in a fixed, tangible form. To express this in another way, copyright does not protect ideas, concepts, stories in an of themselves. It only protects the particular way in which these things are expressed. Thus, there is the need for a story, for example, to be in some fixed, tangible form. So, if he just told you a story it would not be protected.
In addition, before someone can commence a copyright infringement
case, that person must register a copyright with the U.S. Copyright Office. If this person has no registered copyright on any work, you could move to dismiss the lawsuit.
If you had some sort of contract with this individual that you would not use, exploit or disclose the story, he could have a breach of contract claim against you. But I take it that there was no such contract with this individual.
So, I don't think he would have any basis for any lawsuit.