Might be your son is having Allergic Rhinitis
which is mostly seasonal.
Usually it presents with running nose, red itchy eyes etc without fever
You may continue Elriz syrup alone for about 5 days. And for the blocked nose you may use saline nasal drops (Nasoclear) 2 drops in each nostril
three times a day for 4 to 5 days.
If he develops the same again and again, ask your doctor for medicines that prevent allergic episodes.
Hope my answer helps you.