I agree with fluffy, animals are very good at not showing obvious signs of illness until they are really very unwell. I take pets to see the vet at any change in behaviour or demeanor. This sounds like something that should be checked out quickly, in case he has an infection.
I think my cat might be ill but nothing I can really put my finger on. He seems almost depressed, no real life in him. He's only 3. He is still eating but not with his usual gusto. And what seems really odd is usually my dog chases him round, it's a game with them. But dog just keeps following him around and whining. Even when the cat is asleep in the dogs bed she just lets him sleep there rather than growling at him.
About 3 weeks ago he was in a fight with another cat and I took him to the vets. He had a couple of puncture wounds (now healed) and got antibiotics and painkillers. A few days ago he was sick and it was like spaghetti, I'm guessing worms. So I wormed him, he nearly scratched my hand off. This from a cat who is always so docile.
Does it sound crazy if I take him to the vets just to say I think he's ill?