If he can keep fish down, this rules out the problem being due to a gut blockage. Perhaps he is allergic to something in those other foods. If he does all right on fish then I would keep him on that for a while. Then just try a little bit of the other food and see what happens.
My Cat Is Being Sick After Eating Dry And Wet Food But Is Okay After Eating Fish What Could Be Wrong?
My cat does this too after taking him to the vet it turned out he only had a sensitive stomach. As long as your cat looks healthy and can walk and is lively your fine. Keep giving him fish. If he is sick after fish for a few days take him to the vet as he could have a kidney condition. Best wishes
Probably something in the cat food.