Minimum 1 weeks notice for less than 2 years.
I guess he hasn't signed a contract but that shouldn't matter, there is an assumed contract.
I think the only issue would be the lack of contracted hours. But, there might be something for the fact that he had regularly worked an amount of hours over a period. As if he has an assumed 40hr contract and should have had a formal 1 weeks notice in writing.

Kos should jump in here, union man and all.

I can't see any recourse that would deliver anything but a moral victory, I'm afraid. 

Golden rule of Cider production (or anything): whoever has the gold, makes the rules. I'd look to juvenile revenge, myself.

I think everyone is right, nothing much he can do :(