Most pets experience fluctuations in weight during their lives. Healthy cats often lose or gain five or ten percent of their body weight during the course of a year. As long as no steady trend upwards or downwards occurs, minor weight fluctuations are no cause for concern.
However, if your cat is losing weight continuously, he could have a medical problem. Thyroid disease and diabetes frequently cause cats to lose weight despite a robust appetite. A number of other conditions also can lead to the scenario you describe.
I recommend a vet checkup for your cat. Your vet can confirm that he’s losing weight, and then, if necessary, run tests to ensure no medical problem exists.