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Hi! Sorry to hear about your cat. Hair loss can be caused by a variety of issues from food allergies, thyroid issues, or parasites. It would be best to have your vet examine him to see the exact cause. Have you changed his food or his surroundings? Is he an indoor or outdoor cat? What is his age? All of this needs to be factored before you can determine the cause. I did attach a good article that helps explain all the numerous causes.
It can be happen because of many reasons like change of food, change of living place, allergies etc. But the best way to know that what is the problem in your cat is you should consult with the vet. Only vet can tell you the exact problem.