Hi Diane,I do not have any exp with a teenager but i do have exp of me when i was one.

I would miss school ,sleep in,drink loads of coke and stay up late.Always an excuse why i was not going to school, i would fight with my mum all the time.
I was depressed and had bad anxiety,do you think this could be it? I was suffering inside , but as a kid of that age as well i was not going to talk to my family about it, i was ashamed i was hurting , i wish i could hav spoken to mum but at that age i wouldn't and couldn't.
I think back now how hard it would have been for my mum so i can someone what understand how hard it is for you.

Maybe it is depression ,anxiety is common at that age with school and new things etc.
It maybe drugs.
It could be chronic fatigue but i doubt it as i would go more towards depression ,he may have lacking in iron or some form of vitamin that is making him more tired.

Just some of my exp for you ,big hugs xx