CODA is one of the biggest agencies -
http://www.codaagency.com/home.html
If your brother thinks he has the kind of music that they may be able to represent (he will need to look at the bands the different agents represent), he should send a demo CD to the agent concerned.
The order of tracks on the demo is important, the first one being the one that draws the listener in and that should define the artist's music. Then if the agent wants to hear more, he/she will use the other tracks to get an overview of what's on offer.
The agent I spoke to said that she does actually listen to
all the demos she receives - eventually, because she gets lots. She appreciates a demo that is well presented because, she says, it shows that the band cares about itself. Also (unlike in the publishing world), if a band calls her to check the progress of their submission, she's quite likely to pick it out from the pile and have a listen to it there and then. Again, she appreciates a band that is more likely to promote itself in the event she takes them on. Demonstration of self-belief is vital to her.