Tucked tail and wanting to sit immediately upon examination usually means one of two things. The first possibility is interevertebreal disc disease in the lumbosacral region (the last 1/3 of the spine or pelvis). A partially or even completely herniated disc in the back would not necessarily show up on x-rays, as discs are cartilagenous and as such are not visualized on x-rays. Sometimes with a severe herniation intervertebral collapsed disc space can be visualized on x-ray, but this is not a consistent finding.
With no apparent secondary neurological compromise (I assume your vet did some simple neuorlogical tests on the back legs - reflex hammer, flipping the paws over), treating with pain management and rest is a reasonable course of action. However, you did not mention which pain med was used. In cases of possible spinal or any musculoskeletal inflammation, I like to select a non-steroidal anti-inflammatory (NSAID) that will reduce inflammation as well as pain. Some good choices for use in dogs are: Rimadyl, Metacam, Deramax, and Zubrin.
A second possibility is that the anal sacs are full or infected. For a detailed explanation of this problem, please see the following article I wrote on the topic:
http://www.maybeckvet.com/analsacs.html
Most dogs scoot and lick the anal area excessively when this occurs, however, giant breeds often do not do either of these because of their size and lack of flexibility. With a dog that has non-specific pain, tucked tail, and sitting down when examined, checking the anal sacs would be one of the first things I would do.
My advice to you at this point would be to have your vet check those anal sacs if it has not already been done. If you come up empty there, I would then try treating with a good anti-inflammatory, preferably one of the ones I named above (if I had to pick a favorite, it would be Rimadyl). I would combine this with 3-5 days of Valium (diazepam) to help relax any muscles that may be knotted up from the dog having tweaked the back. Most importantly, strict rest, short leash walks of 5 minutes only to eliminate, and no running, jumping, or stairs - for 2 weeks.
If you come up empty with the anal sacs, and the regimen (or one akin to it) I just described offers no improvement, then the boy should get referred to an orthopedic specialist to be assessed and possibly have some advanced imaging of the area, like CT scan or MRI.
Best of luck to you, and thank you for choosing JustAnswer! Please do not hesitate to let me know if I may be of any further assistance.
Sincerely,
Roger L. Welton, DVM
President, Maybeck Animal Hospital
Author, "Canine And Feline 101"