If library use seems like it'll be an important factor, the Nook. Unless you want your cousin to learn about stripping DRM early, which could be educational in itself.
You can check
Overdrive's search to see if there's a library with e-books available in his area, and if the selection is something that looks like it might appeal to someone his age. And of course there's always the Philadelphia Free Library option.
Also, with the Nook you can pre-load any of your own B&N books you think your relative might be interested in onto it by typing your name/CC# combo to unlock the files. Mind you, Kindle allows the same thing using the registration-swapping trick, so they've got feature parity there.