I agree! Beagles are so predisposed to back pain. Just keep in mind that he can't be on both aspirin + a non-steroidal anti-inflammatory. I'd recommend getting another thorough physical exam done at your vet - if not, x-rays specific for his spine or even an abdominal ultrasound may be necessary!
Keep us posted and good luck!
Anaplasmosis is a tick borne disease which causes fever, lethargy, decreased appetite, and musculoskeletal pain. It is usually quite successfully treated with doxycycline for 10 days, so if your dog's pain persists, I would be concerned about other causes for pain such as a bulging disc in the back or neck (common in beagles), or inflammation in the spinal cord or abdomen. If screening xrays and labwork are not informative, then more in depth diagnostics or referral to an veterinary internal medicine or neurologic specialist would be the next steps.
Good luck,
Kimberly Coyner, DVM DACVD