I remember starting a similar thread actually, back in those days and surprisingly a lot of people said the same: Is he unwell? Take him for a check up'. Well, there was nothing wrong with him.
He was just massively overstimulated mentally, having to remember new rules, where things were, what to do.
It was like coming home from a huge party where you've spoken to millions of people and you just cannot wind down...you have to mentally process everything that's happened all day.
A few months solved the problem for us, when he returned in the autumn he was much, much more able to cope in every way. From remembering to EAT all day, and drink enough, and go to the loo, to just having more stamina.
I always gave him a snack after school, in the car, in case it was just low blood sugar, but it wasn't enough to combat the total wipeout!
Mentally even rather than physically. It's a long and busy day for someone who isn't long past 3 years old.
So I agree with Marge!