You will require a paid-for version of the Minecraft client in order to log in to the Beex Minecraft server [subs, please check], but then your brother would have already had one of those. If you know, or can find out, his Minecraft username and password, you'd be able to download the client from
http://www.minecraft.net without the need to pay any extra.
Owen's stuff is amazing, by the way, and he kept telling us about little secrets he'd put in there for us to find.