Do you mist him?
Chams absorb some of the water they need through their skin.
This should just be a wake up call that your cham isn't getting the kind of drinking stimulation that he needs.
If the dehydration has been going on for awhile, get the cham in to see a vet. It doesn't take long for a cham to perish from dehydration.
If your showerhead has a fine enough setting, think about putting a ficus in the shower stall, and let the water run for about a half hour. (room temp water will feel a little cool to the touch) If your showerhead has too big of droplets, please don't try this. It will just stress the cham out and make things worse.
Along with your regular watering schedule, put him in the shower for 30 minutes a day, so long that his condition is improving. If it worsens, take him to the vet.
Do you use a drip system?
Have you looked into an automatic misting system?
Joe