My cat refuses to eat or drink...all test are normal.?

I have a 4 year old pure breed Bengal cat that has not been eating on his own for about 2 weeks. I had gradually changed his food about a month ago and he liked it. Then he suddenly slowed down his eating to where he refuses to eat anything, even tuna which he would previously beg for. We took him to the vet and the x-rays, blood work and physical exam all check out as normal. He had tapeworm which I gave him deworming medicine about 1.5 weeks ago for. Now we are force feeding him the Hills prescription food AD and he is on Reglan and two other antibiotics. We also took him in for fluid and vitamin injections and still no change. Do you have any idea what could be causing this?