Often vets use x-rays to diagnose a soft tissue injury by ruling out any problems the x-rays do not show. Lucci has been x-rayed head to toe more then once while trying to figure out her on and off front end limp that started years ago.
Do yourself and Butt a favor and make the vet do both ends of the dog while under. This will stop him from having to re-sedate should something pop up in the rear anytime soon and sometimes the "real problem" is not where the limping is. With dogs carrying their weight 60/40 a problem in the rear can quickly cause a problem in the front...not saying that is the issue here just pointing that out.
Great point. Ask for "OFA qualified" films while he is sedated and do both hips and elbows - the vet may also want to look at the shoulder. Go to www.offa.org
to learn about what they require and the application - this is a critical minimum health certification for every large breed and I would hope your pup's parents would have had them. If not, then you do not know what orthopedic problems may be passed on to your pup, and the sooner you x-ray and get an OFA opinion the better to know what your options will be.
How old is your pup? Elbow dysplasia can show up at literally any age depending on severity. Many dogs do not show symptoms until later in life despite living their entire lives with it.
Also neck, skull and back problems can also manifest as a leg issue - often even in the rear so it is hard to say where the originating problem lies. I had a dog who ran into a rod iron fence, getting her head stuck between the bars. She later started showing a compromised gait that looked like the problem was in a rear leg/hip. Her hips turned out good, and ultimately specialists found the original "alignment" issue in her cheek/side skull - fixed that through therapy, then her neck, then her shoulder, and on down her body. Everything flows from the front to the rear so everything was compensating and it showed only in the very rear in this case. Just an example.