Small hyperpigmented bump on my cat's belly?

If it's in the exact center of his tummy, it could be his navel. Cats don't have fur on their navel or their . (Yes, all male mammals have .) However, it would be a good idea to have the vet check it at his next check-up. Keep an eye on it and if it gets bigger or starts poking out more, take him to the vet sooner.