If he is a fixed male cat then you should at least have him checked by a vet, because he is in pain.
Over the years we had many male cats and from the first one we learned that it is matter of changing the food, if you don't do anything about it the poisons go back in the body and they die.
Willow says it is the nitrites but I can't remember since we have not have cats for the last 5 years, but I know for sure that since we put every male cat on special food we never had problems with the next 5 cats.