This is normally an early indication of internal illness and i would suggest taking him (or her ;-) ) to the vet as soon as for a check-up; my dog was the same for a while and we changed his food several times thinking he was just bored of the taste or the like. We booked him in to be neutered and his blood test prior to the operation came up that he had liver disease.
i'm not saying this to worry you or saying this is the problem with your dog, believe me, but just warning you that small changes in your dog's behaviour, especially with eating habits, can be early indicators of underlying problems.