Thomas Gilbert Jr., who has refused to cooperate with doctors seeking to determine his mental stability ever since he was charged in January 2015 with murdering his father in his parents’ Beekman Place, Manhattan, apartment, was given one last chance to do so on Tuesday.State Supreme Court Justice Melissa Jackson told Mr. Gilbert, a Princeton graduate whose family also had a house in the Georgica Association, that his refusal to speak with doctors sent by the prosecution may preclude a defense based on insanity.Mr. Gilbert’s attorney, Alex Spiro, told the court that his client was suffering from the delusional belief that the doctors sent to examine him were not real doctors.Justice Jackson then addressed Mr. Gilbert directly, asking if he understood that “these are doctors required under the law” to examine him.Mr. Gilbert, whose hair is now below his shoulders and who has grown a long beard, nodded yes. “Let the record reflect that Mr. Gilbert nodded his head yes,” Justice Jackson said.Mr. Spiro said that Mr. Gilbert’s nodding should “give the court no comfort” that he actually understood what was going on.Mr. Gilbert is being held on Rikers Island without the possibility of bail. He was taken into custody within hours after his father was found dead.