Japanese husband adopt American son?

We've been married for 5 years and I'd like my husband to adopt my son, he is currently 13, adoption would most likely take place when he is 14.
All the information I can find online is related to an American adopting a Japanese stepchild, not the other way around.
As we will not be returning to the US, I need Japan-specific information.
Thank you.

No problem.Go Ahead.
Consult with nearest city hall in japan.

even after adoption, your son's surname is still the same to the previous and his nationality is still USA.
http://marriage.visaemon.jp/article/14360023.html

by the way, the required conditions are here.
http://www.support-zairyusikaku.com/pico+index.content_id+191.htm
http://www.support-zairyusikaku.com/pico+index.content_id+190.htm
it seems that the required documents may vary depending on case by case.

Here you go.
https://www.google.com/search?q=%E6%97%A5%E6%9C%AC%E4%BA%BA%E3%81%8C%E...

And some English info.
https://www.google.com/search?q=adopting+a+child+from+the+u.s.+japan&r...