Luckily, knowing how bone cancer often looks on X-rays doesn't come from my experience with somebody (human or animal) suffering from it.
I can't see any "fluff" around that in the x-ray your posted, but only in the section where the bones are missing I can see the scales very much. But that depends on the x-ray. The bone is for sure not hidden below something, because everything that is bone or "hides" bone (as it is denser) would appear almost white to white on an x-ray. Dark is where the rays go through very well, and white is where they go through less good to not, like bones, or metal from implants, or when for an x-ray something is marked on the skin with a small metal object like a paperclip. So if that part appears dark, there is no bone "below" and that part is less dense than bone - that bone is mostly gone, i can only see some tiny white points where there might be remnants of the bone. (I got "tons" of spine x-rays done, but luckily not for bone cancer but for a severely deformed spine. And everything as dense as bones, or denser, like an implant or something marked on the skin with sticking a paperclip or wire onto the skin appeared white, too.)
So for some reason the bone is very likely gone, and this would affect his spinal cord as it's now open. Likely the reason for the tail problems. In humans, swelling often appears when the spinal cord is open - like for example in an infant with spina bifida. There is a liquid in it that then leaks and causes swelling.
If that's the case is, of course, something the vet has to tell. It's just of what this reminds me (vertebrae missing, spinal cord open causes problem with moving that part of the body and causes swelling), and of what it reminds me rather not (something "hiding" the spine normally looks different: either bones still visible very much (soft tissue) or brighter than the bones (something made from bone structure)).
Let's see how he does, and what the vet might say.
I also agree, 13 years is an old age - but your dragon seems to be fit otherwise