In papers filed in Manhattan Supreme Court, lawyers for Anthony Marshall ask for dismissal of the charges against him, the New York Post reported.
Marshall, 84, was Astor's only son. Astor, who died last year at 105, became an heiress to one of the great U.S. fortunes with her late-life marriage to Vincent Astor.
"From 1999 through the end of 2003, Mrs. Astor gave Mr. Marshall gifts totaling $14,500,000, plus the right to control $30 million in charitable donations," the lawyers said in their filing.
Before Astor's death, Marshall became embroiled in a legal dispute with his son, who said that his grandmother was not being properly cared for.
Prosecutors say that Astor suffered from Alzheimer's disease and that her son took advantage of her failing mental state to rob her. But Warren Whitaker, a lawyer involved in Astor's will, said that she was alert enough at a 2003 meeting to joke about her son's sexual relationship with his wife.