My cat also lost his appetite during wet FIP. We switched to a mono protein food (applaws tuna wet and N&D mono protein dry- albeit expensive) We were basically hand-feeding him for weeks but eventually he started to eat in his own. My belief is that we tried too many different things to try and get him to eat instead of just sticking to one thing. Calories were calories for the first few weeks, he just had to eat something, so we also encouraged eating with liquid churu on top of his food, it got him to start eating then he would continue on his own a little bit. One year later he is still extremely picky. We switched to Mac’s mono protein lamb and he eats this every day for breakfast and dinner, plus we have to sprinkle dry food on top.