hello and welcome to HCM,
You are very clever in assessing using what we call as the whispering test to find out if he has any hearing loss!!
Any sudden loud noise can affect our ears in two ways, one it can cause what we call the acoustic trauma
to even rupture the drum which may cause this acute or severe pain
and also hearing loss. Secondly it can just cause a shock
to the nerves of hearing in which case the pain is usually not seen but only the hearing loss.
In either case you need to definitely consult an ENT who will perform a full examination and also hearing testing as the whispering test is just a screening test.
Meanwhile till you meet your ENT make sure he is not exposed to further loud noise, avoid head bath (prevent even a small amount of water to enter the ear) and do not attempt to clean his ears
If the pain is very severe then see an emergency doctor if simple analgesic
does not work
Dr Sriram Nathan