posterior shoulder dislocation

A 28-year-old male presents to the ED with right shoulder pain after falling on an outstretched arm two hours earlier. He denies history of prior shoulder injuries. On exam, he is holding his right upper extremity in adduction and internal rotation at the shoulder. He is unable to externally rotate his shoulder due to pain, and you note prominence to the posterior shoulder on exam. What is the patient’s diagnosis? What’s the next step in your evaluation and treatment?