What Causes Stuffed Nose Along With Mouth Breathing In A Child?

hello,
My son is 6 year old. He has stuffed nose for 3.5 month, especially in the evening. Sometime he waked up due to hard time breathing. he sometimes use mouth to breath. He used Nasonex and Reactine for one and half month. But no help. He got inhaler. It doesn't help either. He has Saline nose spray once a day. I also bought Mattress/pillow cover and HEPA air purifier. But nothing helps. My family doctor keeps telling me it is only allergy. My son does sneeze a lot. But my son can not sleep well and mouth breathing a lot. It is hard to see him suffering.
Please help! Thanks a lot.