The sense of smell and taste in cats can fade with age.
As a consequence your senior cat may have less interest in eating. You can make his meal more tempting by warming it up in the microwave for just a few seconds before serving. The food's full aroma will be brought out and this should make it more alluring to your cat.
If he's still not very interested in his food, he may have dental problems causing him pain. Cats with advanced dental problems usually have a strong, unpleasant breath.