In addition to things like sleep apnea, this could be a viral illness (mononucleosis is famous, but there are other "post-viral fatigue" culprits) or even type

In addition to things like sleep apnea, this could be a viral illness (mononucleosis is famous, but there are other "post-viral fatigue" culprits) or even type 1 diabetes (before I was diagnosed, the first sign I noticed was sleeping 16 hours a day). Has your son lost weight recently? There a lot of possibilities but I'd encourage you to not just write this off as "he's a teenager". He didn't just become a teenager in the last week.