Have you tried feeding Purina One Sensitive? the reason I say this is we adopted 2 cats from CP once and we were advised that they both had to eat this as their main diet otherwise they suffered diarrhoea, if we tried to change the food it came back. They were also not too bad with other Purina but would start with sensitive first. I know of other cat's it's helped too - it may not help your cat but certainly worth a try. If all tests have come back ok and he's eating well and gaining a bit of weight it does sound like stress and/or food allergy, maybe he will always be tiny but if he's not digesting the food well then that won't help either. Best of luck.