What Causes Arm Pain, Fever And Neck Pain?

Son had severe pain in arm last night. Took him to Dr. today and he told us son has infection in arm caused by a hang nail that got infected. Put son on antibotics and told to go to ER if symptoms get worse. Said it will take couple days for antibiotics to start working. He now feels feverish and said his head/neck hurt. Should we go to ER or wait to give antibiotics time to work?