First, if you notice that your cat has lost weight, you should take him to the vet as soon as possible. The veterinarian will perform a series of examinations and tests to determine the cause of the cat's weight loss. This may include blood tests, urine tests, X-rays or other imaging tests. Through these examinations, the veterinarian can help determine the specific cause of the cat's weight loss, such as illness, digestive problems, malnutrition, etc.

Second, you should observe your cat's diet and water intake. If the cat's appetite wanes or it starts to refuse food, then it may cause weight loss. You can try offering the cat some fresh, interesting food, or try changing the type or brand of food to stimulate the cat's appetite. Also, make sure the cat has plenty of water to drink, as dehydration may also lead to weight loss.

In addition, you should also observe the cat's bowel movements. If your cat's stool is abnormal, such as mushy, has blood, or has an abnormal odor, then it may be suffering from digestive problems or an intestinal infection. These problems can cause the cat to be unable to absorb enough nutrients, resulting in weight loss. In this case, you should take the cat to the vet as soon as possible for further diagnosis and treatment.

Besides, there are steps you can take to help your cat gain weight. This includes providing the cat with high-nutrient foods, such as wet foods with high protein content or special nutritional supplements. You can also try to provide the cat with multiple meals and make sure the amount of food at each meal is sufficient. If the cat has special dietary requirements or health issues, you can consult a veterinarian for more specific advice.

Finally, you should closely observe the cat's behavior and physical condition. If you notice and realize that your cat has other abnormal symptoms besides weight loss, such as vomiting, diarrhea, difficulty breathing, etc., then you should take him to the vet as soon as possible. These symptoms can be a sign that the cat is suffering from a serious illness that requires prompt treatment and care.

Generally speaking, if you notice that your cat has lost weight, you should take him to the vet as soon as possible. A veterinarian can help determine the cause of weight loss and provide diagnosis and treatment accordingly. In addition, there are steps you can take to help your cat gain weight, such as providing highly nutritious food and multiple meals, but it is best to consult a veterinarian for professional advice before taking these steps.