No, it isn't. There probably is some law he can be prosecuted for, in assisting terrorists or something like that, but unless you've actually become a citizen of a foreign government or formally revoke your citizenship yourself, it can't be taken from you.

(Naturalized citizens can be stripped of citizenship if it's shown they lied on their citizenship application, but that's not the case here.)

Fighting in a foreign army against our troops is treason, but I doubt we'll be able to prove it to the standards set by the U.S. constitution. You have to have two eyewitnesses to the treasonous act. If he has a good enough lawyer, and given the appearance of his father on television he's almost guaranteed a very slick lawyer, he claims he was over there just learning about the Taliban and happened to be in the wrong place at the wrong time when captured and imprisoned.