A 9-month-old boy presents with a history of recurrent hypernatremic dehydration, inconsolable crying, and poor weight gain. Laboratory tests show a serum osmolality of 310 mOsm/kg and a simultaneous urine osmolality of 150 mOsm/kg. An intranasal dose of vasopressin fails to increase his urine osmolality. What is the most appropriate long-term oral medication to help reduce his urine output?