I am sorry to hear you are worried about Bubba. I am a veterinarian, and will do my best to help.
If antibiotics are not helping, then this is probably not due to a bacterial infection. If he is losing weight, and not eating well, and especially in an older cat, I would be looking for internal diseases, such as hyperthyroidism
, chronic kidney
disease, liver or heart disease, or even cancer.
If her were my own cat, I would probably take a chest Xray and do some basic blood tests to check the functions of his internal organs. Have any blood tests been done? If so, can you send me the results?
Cats are so secretive about being sick; they hide their symptoms as long as they can. Not eating well is often the only outward sign of illness, and missing even one meal is an alert to me to look for a problem. If he is older and has lost weight, that is even more of a red flag to go looking for internal disease.
Let me know what other questions I can help with or answer.