What could these symptoms be?

[TheHighQueenOfTheFarRealm]

Teenage boy, slim, active, phone kept downstairs overnight, blood tests are all clear.
Symptoms are -
V tired, sometimes needs a nap after school.
Complains of joint pains and muscle aches in limbs constantly.
Finds it really hard to get up even after a full nights sleep.

Does have GCSEs this year so I can't rule out stress but I don't think this is likely?

Vitamin d levels were fine but could it be another vitamin deficiency?

Long Covid would be my first thought.
And ME

I think there is some research that says young men need to sleep a lot because their hormones are raging. Always around GCSE time/age. It's said this may be the reason for poorer performance than girls in exams