Hi Balajee,
My son also had this problem. You have to take him to an Ear, Nose and Throat doctor. My son had enlarged adnoids, they are in the nose and make it impossible to breathe by nose. It is an obstruction, and my son was always sick with colds almost on a monthly basis. He had his adnoids removed and had tubes put in his ears and once this operation was done he immediatly breathed through his nose, it was amazing... good luck

Yes, you really need to take him to the ENT doctor. Not using his nose is preventing him from getting the most oxygen out of the air. Once you find the problem he may even need to re-learn to breathe since he's been doing this all his life.