Burning sensation in the back can occur due to many reasons. As you have pointed out spondylosis (age related spine degeneration) and disc disease can cause the same, which is likely as he has already been operated on the spine. Diseases like
diabetes if any can cause burning truncal pain radiating from back also.
Next Steps
So it is advisable to consult a neurologist and get him examined. Perform a scan if necessary and proceed with medications, ointments, physiotherapy and life style changes