Help! Kitty with chronic diarrhea and our vet can't figure out the cause.?

Our kitty (16 weeks and a little over 3 lbs.) has had a bumpy start. After adopting him from a foster family, he was diagnosed with a URI and given Clavamox for his sneezing. He got progressively worse and a week or two later, we were told he had pneumonia and was spiking fevers and needed to be hospitalized for IV fluids and nebulizer treatments. Later that week, we were told that he was doing better, but the next day they said they suspected he had FIP and would need to be euthanized. Thankfully, we brought him home to spend a couple of days with us before they put him to sleep because he got better every day, with no fever and the only symptoms he had being diarrhea (2-3x/day) and signs of anemia (licking cement and trying to eat his clay litter.) We switched to newspaper based litter & his antibiotic was switched to Clindomycin and back to Clavamox. He was given Metronidazole for his diarreah, which only served to make it worse. Tests came back NEGATIVE for FIP, FIV, FeLV, Toxoplasmosis, and Bartonella. They said he was anemic and seemed to have liver issues/hepatitis. He is an indoor only kitty and has been treated for fleas and the vet *thinks* he was treated for heartworm and Giardia. (?) His current symptoms are the AWFUL diarrhea, trying to lick cement near the fireplace, and " dragging" like a dog with worms. His appetite and fluid intake is totally normal. The vet even had us change to a diet of canned venison or duck, suspecting a dietary intolerance, but there's no change in the diarrhea. Anyone have any suggestions as to the possible cause or treatment? Our vet bill is in the thousands and we can't afford to keep paying for what seems to be a guessing game. The only other thing suggested was a test for portosystemic liver shunt. HELP!