A few thoughts; Is he overweight? Does he sleep at night or is he sitting up in his room playing XBOX or PS3 or even talking to girlfriend or friends on his cell phone all hours of he night? I am speaking from experience here so that is why I threw those few possibilities out there for you.
Another thing that I have experience with that matches the symptoms is Hypothyroidism
. I have had this disease or at least was diagnosed with it, when I was 11 or 12. I was always tired and drowsy and also couldn't lose weight for anything. If you aren't familiar with this, it is treatable/manageable with medication.
My unprofessional guess is that he's not getting proper nighttime sleep which is translating into the exhaustion that you are witnessing after school.
Also, I suppose puberty
could drain you a little and also the symptoms sound like potential depression
I know you will be the best judge of what is going on with your on son, so by all means if you feel it to be more than video games, internet or cell phones keeping him up at night, maybe your family doctor should run some simple blood work on him and make sure its nothing major. For the thyroid, you can mention to your doctor possibly a TSH
, T3 and T4. These are the blood tests that I have every 6 months or so to monitor mine.
Sorry for the rambling, but your symptoms could mean anything from nothing to something!
Take care and good luck.