Catocony: I'm sure you're right that, technically, a US citizen does not need a passport to enter the US. Perhaps I worried too much. However, I wanted to avoid a lengthy secondary at Logan. And, since my son has no picture ID other than his passports, I didn't want to be trying to prove his American citizenship with an expired US passport. As we all know, immigration officers don't always know or follow the letter of the law and they can choose to make things difficult and time consuming.

GUWonder: Thanks again for the helpful advice, which saved me from a costly and troublesome rescheduling of my trip. I think you're right: The biggest potential hurdle was check-in with JAL. That's why I was so relieved when JAL clearly told me that they would fly my son over on his Japanese passport (with ESTA, of course).

My main reason for posting was to help people who find themselves in the same situation. I wanted to let people know that Americans with dual nationality can travel on their second passport with no problems as long as they have an ESTA associated with it. As GUWonder points out, someone could raise the question of having lied on the ESTA application, but I doubt that most border agents would do so.