What would cause diarreah in a 14 week old kitten like this?

my 14 week old kitten has already been to the vet for diarreah and was put on antibiotics and meds to clear any parasitic infection. the problem is now the diarreah has turned a manilla to gray color and I was just wondering what would cause this kind of diarreah in a cat? if anyone has any experience with this kind of problem with cats and what to do to help my kitty.I just want to know what the problem might be with him to be having diarreah like this? this has been going on for over a month now and I took him to the vet for it 3 weeks ago but now it has gotten worse ( the color of the diarreah, the smell) he is an indoor cat only so what could be wrong with him?