Jardine digestion problem

My Jardine has been admitted to our local vet over the last four days, with some kind of digestion problem. He is eating constantly without getting full. When the problem arose, we noticed that his stools had changed in color from green to beige. He resorted to squawking constantly and was not himself. The vet has checked the stools for parasite infection, and all seems to be clear. Do you have any suggestions?