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]
(..) And give me my scarf back! Lightyear! Explain our overnight accommodations. Sir, yes, sir! Prisoners sleep in their cells. Any prisoner caught outside their cell spends the night in the box. . . Prisoners do not speak unless spoken to. Any prisoner talks back, spends the night... ln the box! We get it. At ease, soldier. They're neutralized. But remember, they'll say anything to make you doubt yourself. (..)2010 Toy Story 3