What are the possible causes of hearing loss ?

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.

Most common cause will be wax in ear. other causes includes perforated eardrum,
- middle ear infection (otitis media)
- external ear infection (otitis externa)
- acoustic trauma .. Some rare causes include foreign body in ear, congenital fixation of small bones called ossicles of middle ear etc.

I hope you will be satisfied with this answer. Wishing your brother a speedy recovery.