Student or Learner
"It's been hell for him, both Piggy and Jack endlessly at him, everyone relying on him."
Is it saying "bothering" somebody?
It isn't completely clear to me, but it seems to be this meaning of 'at someone' - 'Piggy and Jack endlessly nagging at him'. Where 'nag' means 'to annoy, usually by complaining about faults'.
It's a common way to express this idea, although I usually hear it as 'get at someone', which is why I said it isn't completely clear if this is the meaning in this instance. Some examples where it means 'nagging':
My mum's been getting at me all morning about not cleaning my room.
His wife's constantly at him about not putting down the toilet seat when he's finished.
You get the idea