As a differential, the findings seen on the MR could represent an infectious process with extensive osteomyelitis, superimposed septic arthritis and infectious myositis  (?TB); however, given the blastic changes seen on the radiographs this seems unlikely, especially combined with the relatively small extent of soft-tissue involvement on the MR relative to bony, and the clinical demographic of a younger male who is relatively healthy otherwise. 

Neoplastic processes such as leukemia or Ewing sarcoma are reasonable differential considerations, with metastasis much less likely given the age. POEMS syndrome also comes to mind as a differential in a patient of this age with this appearance, but is quite rare and would require other info to be considered more highly. 

This proved to be diffuse large B-cell lymphoma, and responded relatively well to treatment.