The rhinoscopy is what you need--"rhino" comes from the Greek for nose, I believe, so it's a scope of the nasal passages. Not sure why it would take an overnight stay, but they may want to monitor him since they'll have to sedate them. I wouldn't leave him overnight, though, unless they have staff there 24/7.
To help with the anxiety issues, send an unlaundered shirt you've recently worn--your scent will make him less anxious. Also, once they do the procedure, he'll likely be pretty loopy for a while and won't remember most of the day, anyway, so he may not get as anxious as you fear he will.
They might be able to guess if a mass is cancerous on a visual exam, but they should also do a biopsy of anything they find during the scope. That way there's no guessing about it.
Still praying and sending