He might be suffering from Paranoid Disorder or paranoid personality or paranoid Schizophrenia. All these conditions are serious mental illnesses. Follow your Psychiatrist’s advice very judiciously. He should be in continuous treatment, sometimes for life time. Your treating doctor is in the best position to tell you about outcome. His sexual problem may be the part of an illness or side effects of medicines. Always stay in touch with psychiatrist and follow his advice.