There are innumerous causes of hearing impairment
,starting from congenital to acqiured causes.You have not mentioned his age nor any details which makes it most difficult to suggest posssible causes.There are innumerable syndromes associated with deafness
.or it may be simple wax in the ear.If he had ear dischaarge it might be perforated tympanic membrane and hearing loss
due to this or damage to bones of the ear.damage to hearing may occur due to trauma,a fall or hit on the ear or side of head.Viral infections at any age cause hearing loss.if he is born deaf,perinatal infections like jaundice
,or maternal fevers while he was in-utero may be reasons.Nerve
related hearing loss can occur at any age,esp noise induced,older age or viral infections.you need to elaborate more details.