please meet your doctor
From your complaints it looks like that your son is having a acute sinus infection
and this is causing the pain and nose block.
He will need antibiotics for the infection and should meet your doctor for the prescription.
For the time being you can ask him to take tablet paracetamol
500 mg two to three times a day for the pain. Also ask him to take steam inhalation 2 to 3 times a day and oxymetazoline
drops two drops four times a day which will reduce the block.
This is a short term solution and antibiotics will be required. So please meet your doctor.