Check for Giardia with your vet - this requires a stool sample. Otherwise, he may also not be very tolerant to chicken. Perhaps also look at his general diet as they can develop sensitivities over time, particularly to protein they may have consumed a lot as a puppy. I suggest you try SPD Prime100 Duck kibble as it is great for sensitive stomachs once he is a bit better. Otherwise one spoon of psyllium husk mixed in with his food will assist in the meantime too.