Dual citizenship from within USA

Both of my kids were born in Japan and have a Japanese father. I of course got American birth certificates and SSN#'s at the US embassy in Tokyo right after their births so both have had dual citizenship. About 10 years ago we moved to the US and all is fine, it is just that my oldest is soon turning 18 and we are wondering what we should do to retain his dual citizenship with both countries and if that can be done here in the US or will we need to travel to Japan? Any advise will be greatly appreciated.

we are wondering what we should do to retain his dual citizenship with both countries and if that can be done here in the US or will we need to travel to Japan?

He needs to do one of the following two things to remain a dual citizen:

1. Fill out the form choosing Japanese nationality. This can be done at the consulate in the US. This form says he chooses Japanese and renounces his US citizenship. It won't actually affect his US citizenship but he will be signing a false statement. This is supposed to be done before he turns 22.

2. Don't sign anything and just keep both. The only person who can force him to choose is the Justice Minister and they don't force people to choose.

http://www.moj.go.jp/ENGLISH/information/tcon-01.html
http://www.moj.go.jp/ENGLISH/information/tnl-01.html