As I'm not American (let alone from the specific part of the States you live in) I'm not sure what the availablity of exposure to the language is locally, but I have known Americans who were able to study Japanese at high school, so when the time comes for that you might think about looking around at schools that offer it. Other than that, luckily, Japanese has a huge online presence and there is a wealth of materials available for learners. I started with "Japanese for Young People" (a high school textbook) when I was about 15 and I found that to be a nice introduction to the language (the layout is not too formal/boring and the dialogs are interesting), there are also a few books that teach Japanese through
manga* such as
this, that might provid the right level of language instruction and stimulation an 11 year old Japanese learner would enjoy

A slight word of warning:
manga and
anime in Japan are not just for kids, there are comics and cartoons for people of all ages and demographics spanning all sorts of topics and genres, so when picking something out for your son (even a book that teaches Japanese through manga, like the one I linked to above), you might want to flick through it first to see if the content is age-appropriate.
*
manga is the term for Japanese comic books, or more generally any comics drawn in the style of a Japanese comic. They are hugely popular both in Japan and the West, I'm sure your son has either read some in English translation or has watched
anime (that's the TV/movie cartoon equivalent of
manga) such as
Pokémon or
Dragonball Z.