Hello, There most likely is a complication from your son s medication and the fact that he suffers from allergies will give him a dry mouth. Practice good oral hygiene and keep the mouth as clean as possible. Especially before bedtime, make sure a thorough dental cleaning is performed after all medication has been taken. Take all medication properly, as directed. Keep well hydrated. This is very important. Symptomatic treatment for the sores include warm salt water rinsing, anti-inflammatory medication, antihistamines such as Benadryl, and gels containing benzocaine. See your dentist if the white areas and ulcers persist. You may have the condition of an oral yeast infection. This will require a prescription medication such as nystatin. Your dentist may also prescribe a mouth rinse that combines pain relief, Anti-fungal medication and an anti-inflammatory. You may have ulcers due to a viral infection as well. Thank you for your inquiry. I hope your son will feel better soon. I am available to answer additional questions and hope you find my recommendations helpful.