Phosphorus and magnesium are usually the culprits and the severity of an issue with fish flavored/based food can vary from cat to cat. However, he has to eat, so if he were to go on a hunger strike holding out for his fish flavored food, feed it to him and then try to transition as best you can. Personally, I would try to limit or avoid fish in his case as it is generally accepted that it can, at least, be a player in causing the problems.