Two of my good friends are high degree Masons. Actually, they are the same 2 friends who initiated me into the Golden Dawn. Open up Regardie's GD textbook, and right away, it starts on about both Freemasonry and Rosicrucianism. When Rosicrucianism became an official order, only Freemasons were allowed.
In my understanding, from a historical and lineal point of view, Freemasonry sits smack dab in the center of the Western Esoteric Tradition. Without it, we would never have had the Golden Dawn. Sure, they are pretty much just a men's scotch club these days, but the knowledge is still there. While browsing through my friend's Masonic library, one thing became very clear: whatever the members DO is irrelevant. Freemasnry itself, its history, its symbolism, etc absorbed into itself the ancient, pagan Mystery Schools (from Chaldea, Greece, Egypt, etc) and in doing so preserved the esoteric knowledge and symbolism of the West for future generations.
In conclusion, if your brother has an avid interest in the Western Esoteric Tradition, then he can only benefit from being a member.
Also, from what I've gathered, besides being a social club, the development of our moral selves is a main aspect of Freemasonry. So there's that, too. But I'm not a member yet.
Here's some cool articles on Freemasonry and esotericism:
http://www.hermeticinstitute.org/docs/esot-freemasonry.pdfhttp://www.hermeticinstitute.org/docs/chamber.pdf