Hi there, an infection in the arm from injecting drugs is very concerning to me after reading your question. I would take your son to seek medical attention immediately without delay, as he may require hospitalization for IV antibiotics if the infection is in his bloodstream, or even surgery if there is an abscess. Infections can progress very rapidly and this could be life threatening. Do not delay in going to an emergency room to be evaluated by a physician in person immediately. I hope this answer is helpful to you.