I LOVE chams who will drink from a syringe! Saves a lot of guessing how much water they are actually ingesting daily. And, interacting with them is a pleasure. When you offer him water how long do you let him drink? When they are "full" they will tend to tip their head straight up more and more and eventually turn away from the water before they "spill". Chams don't have a sphincter between their stomach and their esophagus that prevents "backflow" if they are too full. And, they have relatively small stomachs. We feed them fewer but larger feeders every day than they probably get in the wild so a smaller stomach is more of an issue. I'll bet a wild cham spends a lot more effort trying to fill that same small stomach. But, small food and lots of moisture is available all day every day in most seasons so they don't need to be super efficient at conserving either.
In captivity in the typical human house, humidity and moisture are controlled and a lot more limited because we don't like living in a rainforest sauna. Moisture balance becomes an issue. What is your cage humidity range? If your cage is at a correct humidity level your cham won't dehydrate between good drinking sessions once or twice a day. All the moisture that's available during the day when you can't offer water by hand should keep him from losing ground. If the cage/room is too dry he will get thirsty sooner and show other signs of dehydration (sunken eyes, sunken casque, hard sheds, skin tenting, hard dense urates, constipation, and orange urates. A little yellow tinge in urates occasionally is a hint to check things again...not a full blown crisis.