Hi,after the operation of Adenoids,congestion might occur in the nose. During the operation,a tube is passed either through mouth or nose for giving oxygen.So,if the tube has been passed through the nose,that might have caused nasal congestion. Congestion in the nose might cause Sinusitis. This might be a reason for headache in your child. He might get relief by using nasal Decongestants for pediatrics(like Otrivin pediatric nasal drops)Steam inhalation through nebuliser,and Analgesic(like Syrup Ibugesic).In most cases ,the headache should get relieve in few days. Hope this answers your question.Take care.