In trouble!

Can you inform the doctor who did the minor surgery this is happening, or did you? I would also talk to his GP looking for advice. You are correct that something is very wrong.

I'm with Tetra. Something wrong. Might be mental rather than physical - but still wrong. Please talk to his GP or whoever is in charge of his post op care.

I'd be talking to his GP even without his consent, I could understand sleeping 22 hours once or twice a month if he isn't sleeping normally otherwise, to me 18-20 hours regularly is sending off alarm bells in my head.

debrajo, “hypothyroidism, sleep apnea causes excessive daytime sleepiness,” www.alaskasleep.com/blog/the-thyroid-sleep-connection
Also,
“One clinical consequence of overexposure to bromine is suppression of your thyroid, leading to hypothyroidism. Another is bromide toxicity.”
“You are already exposed to far too much chlorine and bromine. Bromine can be found in a number of places in your everyday world, including:...Medications, such as Atrovent Inhaler, Atrovent Nasal Spray, Pro-Banthine (for ulcers), and anesthesia agents”
articles.mercola.com/sites/articles/archive/2009/09/05/another-poison-hiding-in-your-environment.aspx

Get a test to check his iodine level and for hypothyroid.