First, I assume you do not owe him money. There may be lien rights in your state which could allow him to hold your documents until you pay him if you owe him money. If that is not the case, I would contact him and threaten him with legal action or indicate that you have hired an attorney. A simple letter from an attorney should get him to return your tax documents. You may also want to consider filing a grievance with the regulatory body that oversees the conduct and accounting licenses of accountants in New York.