If your cat is losing weight and  not just fur, it’s important to get him checked out by a veterinarian.  Although stress is one of the main causes of weight loss in cats, there could be a host of other reasons why your cat is losing weight. These include, parasites, heart worm, cancer, feline leukemia virus and kidney disease are only some of the potential reasons for weight loss in cats.  With so many possible causes, there’s no sense in guessing what’s wrong. You need an official diagnosis from a veterinarian. Take note of other symptoms. Is your cat eating?  Did he stop playing with his cat toys?   Does he have diarrhea?  How has his behavior changed?  Observe your cat carefully, then take him in for a thorough exam.

I once had a cat who started to get thin very fast. He stopped eating and became listless. It turned out that he had eaten the tinsel on the Christmas tree and got a very severe infection which almost killed him. Luckily, I was able to spend time and slowly nurse him back to health.