You have two options...take him to a vet...or potentially, watch him become even sicker.
If his eyes are sunken...he is dehydrated. You may be seeing him drink...but it still may not be enough.
Do you see him sitting with his mouth open? Or do you hear popping or crackling when he breaths?
My cham had two back to back respiratory infections, and was extremely dehydrated.
I had to force feed him, and force him to take water as well...and there is no way for me to know if his dehydration led to his illness, or if his respiratory infection led to his dehydration.
You really need to get him checked out by a vet. Sooner rather than later. Once you've done that, you can also try fluker's repti-aide, for insect eating lizards. It helps to get nutrition and fluids into the lizard in times of stress.
Good luck to you and your cham....