I am sorry to hear about Buckley being so uncomfortable. My name is Rebecca. I am a veterinarian and will do my best to help.
Panting, trembling, and keeping the tail tucked are all signs of pain in dogs. Beagles are prone to getting inflamed and bulging discs between the vertebrae, causing back pain. Often when this happens, there is no accident or injury; it just happens.
If this is his problem, the treatment is usually a combination of muscle relaxants, pain and anti inflammatory medications. Most over the counter pain drugs and muscle relaxants are not safe for dogs (Flexeril, Ibuprofen/Advil, Aleve, are all dangerous in dogs), so usually we use prescription medication.
Until you can get him to a veterinarian who can prescribe things like carporfen, tramadol, gabapentin or methocarbamol, you could try baby aspirin. How much does Buckley weigh? If he is not vomiting, you can give 1 baby aspirin, or 1 low dose aspirin, per 15 to 20 pounds of his body weight. That can be repeated twice daily, and it should be given with food. If he vomits, do not give any more aspirin.
Let me know how he is doing and what other questions I can answer.