Any Linux distro will do fine, as all of them (leap of faith) provide g++ compiler or a way to install it.
Personally, I like to work on Debian for servers and Ubuntu for workstations/development. However, since most of the stuff is ran on the servers, my workstation is mostly a ssh client and all the work is done on Debian.
I think Ubuntu will be fine for your nephew, just make sure he gets the LTS version, you don't want him reinstalling a development computer often.