Hello Linda,
Well it sounds like to me that he may be developing the early stages of a upper respiratory infection, or other medical issue. Birds mask there symptoms, meaning that they will not exhibit any clinical signs till the very end. This is based upon flock behavior. Basically in a flock a bird that exhibits illness, is not allowed to eat, drink, or socialize with others. Eventually this will lead to the bird being picked off by a predator, or another form of death. I know this sounds very odd but birds still exhibit this behavior in captivity.
In canaries one of the key signs is lack of vocalization. Canaries sign when they are stimulated, happy, and healthy. You mention that there has been no change in husbandry or environment, so that usually will leave only viral/ bacterial infection.
My reccomendation would be to seek the assistance of a qualified avian vet asap. They will mostly likely do a brief physical exam, and place him on oral medications. Most likely the medications they will place him on can be added to the water for a period of 14-21 days.
Good luck, and if you have any further questions or need help locating a qualified avian vet let me know.
John