This happened to me before. In my case I sold the car to someone, they never registered it. Owner abandoned. Was my responsibility to get it out of where it was left as according to NYS DMV, I still owned it whether I had the title in my posession or not.
Plain and simple, you are responsible for it. Reporting it stolen is not the best idea in this situation. The right thing is for him to pay you and for him to get it out of there ASAP.