If I am not mistaken, US citizens should enter/exit US using US passports and/or citizen land @ POE. Even without his US passport at hand, your son shouldn't enter the States as a visitor.

Of course your son will tell the truth. @:-) I am not sure what would satisfy as the proof of his citizenship w/o his US passport. I'll leave it for other FTers to chime in. I don't think US can deny its own citizens to enter the country though. See if you can find some helpful info from this thread about using dual passports:

http://www.flyertalk.com/forum/trave...questions.html