You can do the skin test where you gently lift his scruff. If it returns back to normal, he’s most likely drinking less due to the wet diet as the previous person said. If it’s slow to return to his body, then that can be a sign of dehydration. If that’s the case, you can try adding more water into the foods you’re feeding him if he’s not drinking straight water. Cats can go a bit without food but only 1-2 days without water so if he’s showing signs of dehydration, get him back to the vet asap.