While this patient's back pain is likely musculoskeletal in nature given his history of IV drug use and low grade temp - with no other localizing source - epidural abscess is also in the differential. An MRI could be ordered to rule out this entity but a recent paper in Journal of Neurological Spine suggests that an ESR, if normal or very close to normal, may be just as good at ruling out spinal epidural abscess in this patient.
Click here to view a post by Dr. Michelle Lin on a reasonable approach to ruling out spinal epidural abscess.
Source
Lin, Michelle. "Paucis Verbis: Spinal epidural abscess" Academic Life in Emergency Medicine. 6 Aug 2011. http://academiclifeinem.blogspot.com/2011/08/paucis-verbis-spinal-epidural-abscess.html
Davis DP et al. "Prospective evaluation of a clinical decision guideline to diagnose spinal epidural abscess in patients who present to the emergency department with spine pain" Journal of Neurosurgery Spine. Jun 2011.