Has he had any changes or upsets to his routine lately? My brother's Siamese Cocoa did the same thing. It started when I went for a visit for a week, and then stopped when I left, and he had also just had minor surgery on his ear. The vet says it wasn't anything medical related to the anesthesia or meds after, so we came to the conclusion that it must have been my presence there, since it stopped when I left. He may have been upset or jealous that I was perhaps paying more attention to one of the other cats, than I was to him. That's the only explanation for the problem that my brother could think of. Oh sure - blame me!!!