I am sorry to hear you are worried about Charlie. My name is Rebecca. I am a veterinarian and will do my best to help. I just came online and saw your question. How is he doing? I am sorry you did not get help from an expert until now.
Panting can be a sign of fever, pain, stress, or difficulty breathing. If you check his gums, are they pink in color, or do they seem pale? If his gums are pale, he may be having trouble breathing, which could be from a heart problem, fluid in his lungs, a blood clot in his lungs, or an infection.
Can you take his temperature with a thermometer? Normal is between 101 and 102. If he is over 103 that is probably the reason for the panting.
I see he is 13, so that makes me more concerned this could be serious. The best place to start would be a thorough physical exam by a veterinarian: listen with a stethoscope, feel his pulses, palpate his lymph nodes and internal organs. Until you can get him checked out, keep him quiet, and continue to get him to take liquids like milk, water, broth, or Pedialyte to stay hydrated. If you think his gums look pale at all, this could be an emergency.
Let me know how he is doing and what other questions you have.