Hi My Son Is Experiencing Sinus Problems Right Now. He

Hi My son is experiencing sinus problems right now. He is plugged up on both sides of his nose and sometimes it changes and is on one side and then switches and goes to the other. His saliva is constantly building up and seems like a lot and if swallowed gives him a sore throat. What could this be from? Thank you - he has no aches, pains, fever or cough.