Yes this seems as a typical case of otto toxicity from prolonged use of gentamicyn. It is well known that when the gentamicyn is used more then a week especially in high risk patient (as your father suffering from meniere syndrome) the otto toxicity and renal failure
are the most common side effects.
I recommend that your father should go to a ENT specialist
and perform the Audiograme, and also the creatinemia and blood urea should been check.