What medication is given to your boy?
It's quite common to have stuffy nose especially in air con room because their nasal passages are small and still developing.
My boy uses Otrivin (Illiadin can also be used). You can get over the counter from any pharmacy. It's safe to use on infants. Buy the infant version.
Use 1 drop per nostril twice a day. Stop after 3 days, rest for 1 day then start another 3 days if symptoms persist.
Having said that, it is better to clear with the doc that the blocked nose is due to a sensitive nose rather than an underlying illness.