I had a supervisor who fell asleep all of the damn time! I never said anything to him and later found out he had been fired from the centre because he was falling asleep all of the time. Anyway he was about 60 but had very bad diabetes. If it was my t I would be really annoyed. You sound like a patient and kind person and I really don't like the way your t is denying it, that would annoy me more than the falling asleep. I understand he is old but could he try to prevent it by engaging with you more and making a conscious effort to stay in contact with you when you start monologging