Thanks so much for your very helpful reply.
It sounds like he has hyperthyroid. That is very common in a cat his age.
It is not true that nothing can be done. This ailment is quite treatable with a food made by Hills called Y/D.
It comes in wet and dry forms:
In the meantime you can help him put weight back on by feeding him small meals every two hours. That is the best way for his body to metabolize calories.
He can have tuna, salmon, canned food, boiled chicken,baby food (no onion or garlic), anything he likes and will eat.
If you use dry food feed grain free as he is less likely to throw up from that.
Please let me know how he does on that program.
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