He's entitled to "ask" anything he wants. You are entitled not to answer. You don't have to tell Uncle Sam what you were doing in Japan if you don't want to. They still have to let you in if you have proven that you are a US citizen.
They can't deport you for refusing to answer. They can't arrest you for refusing to answer. They can't keep you at the airport forever for refusing to answer. All they can do is yell at you and stamp their feet and threaten you and search your bag and then let you go home in 1-2 hours.