It's perfectly normal for your cat to shed regularly; but if your pet has bald patches, red and inflamed areas without hair, or spots that are so thin that you can see the skin below, he has a problem. Cats can lose hair for problems as minor as a scar or for more serious issues like skin fungus, mites, fleas or a hormonal imbalance. Inflamed skin, or dermatitis, can cause hair loss; stress is also a factor in excessive shedding. Some cats get so anxious that they lick or chew their fur out.
If you notice the hair loss is coming from certain raw patches, or hot spots, your cat might have fleas or mites. To ascertain the cause, check your cat for parasites. If the hot spots are a result of a flea-bite allergy, you need to get rid of the fleas and keep them away so that the hair can grow back. But if the spots are in places where your cat can't reach in order to scratch, the cause might be an allergy caused by something your pet touched, like new carpeting. If you notice your cat scratching or licking a spot too much, don't scold him. That will only serve to increase his stress and make the problem worse. Instead, try to modify his behavior by distracting him with a toy.
Hair loss can also be a result of improper nutrition, so check the food your cat is eating to make sure it has everything it should. There's a reason that better brands of cat food are a bit more expensive. If your cat's hair loss is accompanied by other symptoms that could indicate disease like fever, appetite loss, weight loss or vomiting, you should take him into the veterinarian as soon as you can. Some of the parasite and fungal causes for cat hair loss can be contagious to people, too. So have your vet take a look.