You have not given a lot of information, such as if your cat goes outside, his vaccine status, or if you have taken your cat to a vet recently. Answers to these questions and others would help us to help you and your cat.
There are many reasons for a cat to lose fur and weight. Common causes include fleas and flea allergy dermatitis, which can cause excessive hair loss, but you would be aware of scratching or chewing at the fur and skin. Fleas also carry tape worm which could cause your cat to lose weight. Other causes are food intolerance/allergy, which cause skin issues and gastro-intestinal symptoms. Endocrine disorders and auto-immune diseases and infectious diseases (skin fungal or bacterial infections) and other skin or gastro-intestinal parasites can also cause the symptoms you describe, amongst many other causes. Therefore you really must take your cat to the veterinarian for an exam and diagnostic procedures.
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