Need a new multiplayer board game for my brother.

[Driderman]

I like the games from Flying Frog Productions.
Fortune & Glory (Indiana Jones-style pulp adventures where you hunt artifacts and fight Nazis, cultists and mobsters) has both Co-op and competitive rules. Same goes for A Touch of Evil (monster-hunting in a "Sleepy Hollow" american village somewhere in the 1700-1800s) and Last Night On Earth (zombie apocalypse survival scenarios) has a mechanic where one player plays the zombies and the other(s) play the survivors.
The alien-invasion one I haven't tried, but I'm assuming it's as awesome as the rest of them.

All the games support 5+ players, although only Last Night on Earth (and possibly the alien invasion game) has any sort of map building. Still, with the amount of item and event decks to draw from, the games all have fairly high randomization element.