Son completely screwed over. Zero hrs contract. Advice needed please?

[UnFuckingAcceptable]

A local company. He had worked there almost a year.
Won't go into all the dodgy goings on there as that's not relevant to us right now.

No contract given or signed. No employee handbook. Shifts and communication etc all done via group chat on WhatsApp.

Son swapped shift with co-worker in order to go to an event. Shift swapping is encouraged.
Day prior to event son is paid.
Only received pay for eleven hours.
Had worked forty.
Given a dressing down on group chat when enquired about wage as need money for event.
They then sent him a private message saying ' we know you are ok but rest of staff are taking piss. Come into work, will pay you from till but you'll need to give it back in wages (? I'm not sure what they meant here)
Boss continues private message ' I'm going to go in on you on group to make point to other staff, go along with it'
Son not happy to do that.
He doesn't want to be made to look like he's daft on chat with all his colleagues.

He attended work place for the cash for event. Boss decided to dress him down in middle of restaurant as he wouldn't 'play along' in group chat.
Son is very non confrontation, quite laid back and can be timid. He didn't react other than to ask for wages.
Told if he's going to drop shifts to swan off to event, not to bother coming back.
Son left and is now out of pocket to tune of approx £200.

We've sent recorded delivery letter asking for wage and stating hours worked.
We have all emailing from him being offered job and all WhatsApp history.

Is there anything I can do here?
He wants to leave it but it's money he can't afford to not have.

[flowery]

He can make a tribunal claim for non-payment of wages, which is actually pretty straightforward, compared to most claims. I would suggest he gives Acas a call, they should help him with the procedure necessary.