Thank you for your query.
It looks like a viral bug.
Treat your child with nasal decongestants and syrup cetrizine
once a day for 5 days.
Your child needs a Serum IgE test,Xray-PNS to be done.
If they are abnormal then he should be treated accordingly.
I think he has a allegy problem.
Consult your doctor if the problem persists.
If you have any further queries you are free to consult me.
Hope this answers your query.
Wishing a speedy recovery.
Have a nice day.