Hi Mrs. Alifahning,
Thank you for posting your query on healthcaremagic.com and giving us this opportunity to clarify your doubts.
is notorious to cause skin rashes, specially if he is already predisposed to allergies. But do not worry, if the rash is not increasing in size and not disturbing him, he should be okay. If there is any breathing difficulty or swelling you see anywhere on his body(specially the lips, legs, face,etc) you might have to rush to the ER. In any other case, you can see your doctor tomorrow and he will need some kind of anti-histaminic and change of the drug.
The rash can also be a part of the illness , since many viral infections in children are known to cause such kind of a rash.So, just keep a watch on the warning signs i just told you and if in doubt its always better to see the doctor. Kindly do not give the next dose of azithromycin before seeing the doctor.
Hope i have helped you to clarify your doubts and i will be always available in case of further doubts. Take care and stay healthy.