Hearing loss due to beta blockers is very much rare.but there are articles where there is profound sensorineural hearing loss which got reversed after stoppage of drugs.
You have to review your family history,occupation other medication which you got during any hospital admission.
You can go for BERA test and otoacoutic emissions test.
MRI is for any unilateral hearing loss.In a bilateral sensorineural hearing loss,most of the time there will be normal MRI brain scans.anyway MRI can be done to rule out any intracranial causes.
Next Steps
he can go for OAE and BERA tests.and then radiological tests to ruleout other causes.if there are any occupational causes,he has to change his profession to prevent further hearing loss.