Hi there, it is the time of year when many dragons brumate so this may be what's going on. And as far as sand or sand/soil, I've used both in some of my tanks for almost 25 years, never had a single problem.
Just wondering though, do you have a coil/compact uvb ? If it's a strong one it can give off decent uvb but only in a small area. That can cause lack of appetite if he doesn't get enough uvb. And then it may be just a phase many adult dragons go through.