I'm thinking with your senior kitty his teeth could be bothering him, and the dry food may be difficult to chew. My kitty is 11 years old and has had two dentals recently with infected teeth. Have you tried some wet food like Fancy Feast or another brand just to see if it makes a difference? Try mixing some of his usual dry brand with the wet and see if this helps.

Catty has given you some good advice. I hope you can get him to a Vet.