Is he dehydrated? (Poops are a good indicator of dehydration) If the poop is normal, then he's getting enough moisture through the other sources you are providing. If the poops are tiny/dry, I'd be concerned. A little apple, grated carrot, or dark leafy greenscan help provide moisture to the diet.
I advise giving a little Nutri Cal for cats/dogs. Very therapeutic, and has electrolytes, vitamin e (good for the gut) and provides glucose (keeps body from digesting muscle/cardiac tissue by providing the brain with the glucose it needs). An added benefit is that you only have to give a tiny amount and don't have a big mess.
Pam