He was captured by rebel forces and kept as their 'prisoner' for several months before they set him free.
They 'took him prisoner'. = He was 'taken prisoner'. [=they captured him and made him their prisoner]
a 'political prisoner' [=a person put in prison because of his or her political beliefs]
(..) Yes, a bubble. - Do you mind? May I? - Oh, sure. It'd break your heart seeing him in there. He's like a prisoner. No friends. Just his mother and me. And I'm out there six days a week hauling Yoo-hoo. We have sacrificed everything, all for the sake of our little... (..)1992 TvShow: Seinfeld Title: The Bubble Boy Season: 4 Episode 7