Are you able to specify one or more of these reasons? I don't know him or anything but I can't really think of a good reason that it shouldn't be his choice to join these forums, assuming he's 13 years old or over and is willing to follow the community's rules and guidelines.
Super Mario Boards
Supergirl
Jackson B. Humorus
I dunno, as someone with siblings, them being his little brother is a good reason.