My wife and I just got naturalized and have our American passports, but my (minor) son has the British one only, until his N-600 completes. We need to take a trip to the UK before his papers go through, so I am wondering if he can use his British passport to come back to the US. I understand that because of our new US citizenship now he too is a US citizen, but is it illegal for a US citizen to enter on a non-US passport? How shall I do this?