My Father Has Probable PMR, He Was On Prednisone For

My father has probable PMR, he was on Prednisone for 2 years which did bring his CRP & sed rate back to normal. Now he is having radiating pain from his neck to shoulders and upper & lower extremeties. MRI of C-spine show mid encroachment of spinal cord, Neurosurgeon says he does not need surgery to correct. Rheumatologist says he does not think the pain and weakness in his limbs is related to PMR and may possibly be an occult cancer. They are now sending him to a pain clinic. If he even bumps his arm it feels like it is broken and the entire limb very painful. What other potential diagnosis should be in the differential?