Were any photo's taken during the course of the job. He may just be an old crank angry that you got his car dirty. Wait to see if you get a solicitors letter, then worry. In the mean time. Would the person you did the job for i.e his neighbour not have a chat with him to let him know that over spray is unavoidable, an act of god so to speak. You should have been conscious of his wheels though mate and rinsed it down a bit for him.