Welcome to the HeathcareMagic forum
The problem could be due to the ototoxicity
in the inner ear
or sudden sensori neural hearing loss
(due to damage of the sensitive auditory nerves in the ear).
Steroid would have definitely helped in improving the hearing as your father is suffering from diabetes, steroids can't be given. Other alternatives are betahistidine (a peripheral vasodialator) and vitamin B-complex for a month. If he is not improving better to go for the digital hearing aid
in the better ear.
Hope this answers your query.