A 67-year-old man has become withdrawn and stopped eating over six weeks. He says his organs have rotted and that his family will be financially ruined because of him; yesterday he collected his sleeping tablets and told his wife that she would be better off without him. He is markedly slowed, has lost 8 kg and cannot identify any reason to live. There is no evidence of delirium or a substance-related disorder. Which one of the following is the most appropriate treatment strategy?