Did they say what he has? A doctor would not prescribe antibiotics for a common cold/virus because antibiotics simply do not work on the common cold/virus. However if he has bronchitis or pneumonia or a sinus infection, he would need the antiobiotics because that is a bacterial infection which needs antibiotics to clear up. Another person wrote that zithromax is taken for 5 days but continues to work for 10, and that is correct. If the fever has stopped it suggests that the infection for which the antiobiotics were prescribed is getting better. I would definatley suggest a follow up with his regular pediatrician in the next couple of days though so they can confirm that everything is clearing up properly.