Hi,
Your husband must be having sudden sensori-neural hearing loss, it can be confirmed with audiological evaluation and thorough ENT examination. Consult your ENT specialist at the earliest, advise him to take bed rest as well consume fruits and vegetables.
Regards
OHNS2010
It might be that he has a blocked Eustacian tube from a sinus related issue. If he's regularly seen by an ear, nose, throat doc, that is who I'd go to in his shoes.