One of my cats who at the time was an indoor/outdoor girl became quite ill a few years ago and she vomitted for 2 days and then became very listless. I got her to the vet and they said she was going into hypothermia. They put her on warm fluids and rushed some blood off to the lab. Her white cell count was through the roof but they couldn't find what was causing her problem. They did multiple in clinic tests for different cat diseases that all came back negative. They did xrays, an abdominal ultrasound and then exploratory surgery a few days later when she wasn't getting any better. They even opened her bladder because they thought they saw something on the ultrasound that looked a little suspicious. Turned out to be nothing. They decided to put her on an antibiotic drip at the hospital because they didn't really know what else to do and she began to slowly come around. I took her home and continued the antibiotics for the next 2 weeks and she was back to normal. They never did figure out what had made her become so sick. Thankfully, years later she hasn't had another occurance like that.
It sure is strange to read about all the posts regarding 'mystery illnesses' and cats. In any case, I hope your Panda starts eating soon and that he is back to his old self ASAP.