You might like to think that the boss would ask him to stay regardless but your brother should certainly ask anyway. What's he got to lose?
I'd agree that you need the right environment to raise the issue and that it wouldn't be productive to "ambush" the boss if he/she was busy. However, he could he at least tell the boss that there's something he'd like to discuss and make an appointment to speak privately (somewhere outside the premises if necessary).
Don't forget that the boss is effectively getting free labour at the moment so the decision to keep someone on, even at the minimum wage, wouldn't be taken lightly.