Canned food has a higher water content than dry (it is not uncommon to see water or broth as one of the first two ingredients on canned cat food) and the cat may not need as much fluid as if he were on a dry diet or primarilty dry diet. Keep offering fresh, clean bowls of water and make sure you are not leaving canned food down all day (it can spoil). Since you are adding water to the diet, the cat may be getting enough water. If you have worries, contact your vet. Lack of water can result in some health issues.