new ESA query

I applied for ESA for my ADD ( ADHD) son and he has a work related interview coming up next week. He's already received some back pay into his account, but no letter telling him which category he is in or what the weekly award is or how many hours he may work without it affecting his benefit.

He is enrolled at uni but due to health issues he has not been attending so not clear if he will be continuing, but for now assuming he remains enrolled at uni, if he gets a part time job, how many hours per week can he work and what is the upper limit he is allowed to earn without it affecting his entitlement? Would be grateful for clarification, thank you so much.

PIP was refused but we have the appeal date coming soon; he is young and wants to be active but the reality is he doesn't function. He asks how many hours can he work but that's because he wants to. He's lost countless jobs because he can;t keep a job .

sorry forgot to add he hasn;t attended uni due to health and they are advising he come back next year when in better health; will this impact upon how many hours he is allowed to work? I only ask because he asks - even though he can't work.

just wanted to add he's received a letter saying he will receive £52.45 weekly from 19 July. Is that how long it should it take?

sorry ignore my last message - he gets the allowance from July 2016, which presumably means the lump sum payment he just got has been backdated to then. and that he'll continue to get it weekly from now on. As I said he has applied PIP and we have appeal coming up soon.