Single sided hearing loss should always be treated as a medical emergency. My husband has total hearing loss in his left ear from both him and his Dr not realising the potential seriousness of the situation. It went from partial hearing loss to profound deafness within a couple of weeks. If he'd been given high dose steroids at the earliest stage he might not have lost his hearing permanently.
www.nidcd.nih.gov/health/sudden-deafness
I'm sure that 99% of the time it's nothing serious, but it's scary how many people we've met with the same story as dh. Only a little while ago we both went for our annual optometrist appointment and she was telling me about her husband who had the same start of single sided hearing loss. He went to his family Dr who told him that it was probably just congestion. He then went home and Googled, and she admitted that she accused him of being a hypochondriac, but he insisted on a referral. The ENT specialist confirmed that it absolutely wasn't conductive loss, so immediately prescribed the steroids, and he recovered a lot of his loss of hearing.
My DH can't even get a hearing aid for that ear now as there's literally no signal from the ear to the brain. He has terrible tinnitus too.
Nothing showed up on MRI so the most likely theory is that it was "a virus" that destroyed the nerve.
In your husbands case it is hopefully congestion, rather than nerve damage, but the consequences of it not just being congestion are so serious that one should never just assume.
If his doctor has advised "wait and see", without actually testing to see if it's conductive loss or not, then I'd really push for an urgent second opinion.