Strange, it's not clearly noted on the homepages of various Japanese consulates in the US...
There was this written in the homepage of the Japanese consulate in Vancouver.
http://www.vancouver.ca.emb-japan.go...q.htm#visafaq2
Q: My Canadian passport will expire within six months. Do I need to renew my passport?
A: It is not necessary under Japanese immigration law. Your passport must be valid at the time of entry. Please note that airline company polices and restrictions may differ. It is best to confirm their policies directly with them. If you hold a Canadian travel document other than an ordinary passport, it must be valid for longer than six months.
While citizenship is different I don't see much reason why it will differ much from that for US citizens. In which case hopefully your son will be OK.
Hopefully others on this board can provide more feedback.