Your tax records belong to you. If your accountant is licensed (CPA or EA) report him to the licensing agency if he will not return the documents. If this still continues, hire a local attorney to pursue this for you.
Any individual seeking legal advice for their own situation should retain their own legal counsel as this response provides information that is general in nature and not specific to any person's unique situation. Circular 230 Disclaimer - Advice given in this response cannot be used to eliminate penalties with the IRS or any other governmental agency.