I have a suggestion and at least one more test. If your cat has not been tested for Feline Leukemia or FIV then definitely consider this. Also, there is a bacterial infection called Bartonella which is the same condition that causes cat scratch fever in humans. It was once thought that cats did not suffer ill effects from this and they were just carriers. This is not the case at all. Bartonella is transferred from fleas to cats and the cats with this condition will develop mostly upper respiratory symptoms that don't seem to clear with common antibiotics, and sometimes weight loss, diarrhea, and mouth ulcers. The the biggest indicator is the chronic upper respiratory and sinus issues. You can have him tested in which the results will be received in a couple of weeks or treat prophylactically with Zithromax for 21 days. Zithromax is also a great antibiotic for upper respiratory infections that are not related to bartonella and I highly recommend giving this a try before giving up.
Please let me know if you would like more information.
Theresa
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