OtoHNS_MD is right, 20 hours is incredibly lots of sleep, very unusual, even if he's been ruled out for mono and strep, what's the doc telling him? how can he be so relaxed with being so fatigued. i agree, if worse, go back and see the doc.
Is he taking any medications from the doctor?
A bacterial throat infection or mono could both cause these symptoms.
Call the doctor and let them know he's not better (is he getting worse?).