Would be highly unusual to sleep literally for "days at a time". Start with your doc if this is remotely true.
That's probably very worrisome as a parent. Hard question to ask. Do you think he has a substance abuse problem? Addiction can very much look like this. The other concern (and it often goes hand in hand with addiction and substance abuse problems) is mental health. Severe depression can cause someone to be lethargic and stay in bed. Lastly, what about narcolepsy. There is typical and atypical. Both can cause someone to sleep inordinate amounts of time. My sister in law has atypical narcolepsy and she takes medication to keep her up for about 9 hours to go to work and then she is in bed the rest of the time. Literally eats dinner in bed that her husband makes for the rest of the family and doesn't get out of bed all evening once she is home from work around 4 pm. She could do more medication to stay way longer but chooses not to. Anyway, a sleep clinic at a university did a sleep study on her to determine this was the case.
Tell us more about your son, we'll go from there.