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Thank you for your question. My name is Dr. Stacy. I have been a small animal veterinarian for 13 years and I'd like to help with your question.
The behavior that you are describing almost always goes along with mouth pain..... Whether there is an ulcer somewhere in the mouth or stomatitis (a common condition where there is inflammation and pain in the gums) or whether there is a tooth causing a problem. Some of these problems might be easy to see... such as a large ulcer... but others are more difficult and require an exam under sedation and often dental X-rays. How long ago was the previous tooth problem? Do you know if you veterinarian did a full dental exam and X-rays at that time? If your veterinarian can't do dental X-rays then you may need to take him to someone who can. He needs to have a good evaluation of his mouth to identify the underlying problem. I understand that he is 15 years old so the sedation is a bit of a concern, but without knowing exactly what the problem is, antibiotics are not likely to solve the problem.
I hope that helps, but if you have additional questions please let me know.