The trouble with bullies is that they don't stop. Once they find that they can assert themselves over someone, it becomes amusing to them to dominate their victim. It's a power thing.

Complaining to management usually makes the situation worse.

If your brother chooses to stay at his workplace, then he has to either work in another area, away from his tormentors, or simply confront each individual and take them around the corner and give them a good smack in the earhole.
I'm not condoning violence here, but sometimes the only way to extract oneself from a situation like that is to make a connection between your brothers fist and his tormenters mouthes....it shuts them up.

Either that, or he leaves and gets work elsewhere.