You mention a suspected broken arm , but not showing up in xrays .Has he had a fall ?
I had a similar experience many years ago with my then juvenile 6 month old eastern bluetongue skink.
Took to the nearest reptile vet who examined him and told me he thought there was a sprain in his right arm and the swelling should go down in a few days and he'll be fine. This vet even took xrays but saw nothing of any concern.
The swelling got worse and the skink really didn't want the arm touched or to use it, so I took George to another reptile for a second examination and more (better quality) xrays that showed a spiral fracture in the upper arm bone.
The decision was made put George under a general and reset the bone with a specially made in the surgery stainless steel pin and some special biological glue. The arm then had some special dressings applied and was a encased in a full arm and shoulder cast was made to immobilize the arm and shoulder.
3 weeks of antibiotic - antiinflamatory - pain relief injections (every 2nd day) and back to see the vet in a month and the cast removed after 2 months , and George was good as new.
The observation that he's now not using either arm ==> perhaps a shoulder injury (a dislocation and swelling effecting the upper back or neck.
I'd be seeking out another vet for a second opinion and maybe a better xray to get to the bottom of this.
If it's soft tissue injury in the shoulder or upper back , this may not necessarily show in xrays , but there will be swelling that the vet should be able to see.
If the issue metabolic , ie MBD or gout, a blood test will likely be needed to be sure.