I am sorry that you have been waiting for a response. I recently just logged onto the site and noted that your question hasn't yet been answered. I am a licensed veterinarian with over 25 years’ experience and would be happy work with you if you are still needing assistance.
It sounds as if his problem may be spinal in nature and involve an intervertebral disc. They can be very painful and can lead to symptoms of discomfort to ataxia (drunken gait/weakness) to even paralysis. Hopefully your vet was able to evaluate his nerve reflexes as this can help to localize the area of the spine involved. If he did and no abnormalities were noted, it is prudent to monitor as progressive loss of nerve reflexes can indicate spinal swelling and may be an indication for surgery.
You are correct that if a disc is involved, xray are often of little to no help, but can help to rule out other causes of spinal disease or other structure abnormalities. Other conditions can include infection, tumor, fractures of the spine or hips. Some dogs with severe hip changes due to arthritis can develop symptoms acutely and be very painful and can be difficult to differentiate from a dog with disc disease, but an xray would help.
Additionally, if his pain control is not sufficient, they may need to prescribe additional pain medications to help control the pain by a different mechanism and the sometimes synergistic effect of the multiple drugs together allows you the keep the doses of both down, avoiding potential side effects.
Here is a link with more information about disc disease in dogs and it's diagnosis and treatment.
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.