I don't have extensive experience with this particular breed- however I did google in Norwegian Forest Cat - feeding and came up with a lot of web sites - had a quick flick through and there seemed to be lots of advice- found some info on the human equiviant of irritable bowel syndrome in cats. hope this helps and that you find a solution - keep us posted - if just plain chicken suits him iI would suggest that the additives in commercial food don't, or that its too rich for his system. individual animals within a breed often need completely different food.anyway.A personal choice with me(based on years of animal welfare work in UK and Africa) is that I would rather my animal didn't lose essential goodies by having constant diarrhoea than feed what didn't suit even though it was an all singing all dancing feed. would feed something acceptable to his digestive system for quite a while in order to let this calm calm down and then introduce extras very, very carefully. Be interested in the outcome - good luck