Ah, I've got that one worked out. Basically, without a sword he's powerless- or rather, as powerful as a normal person. So he's not in very high security. He managed to smuggle in a tiny knife (because he's able to tell the future he hid it in an area that he knew would be overlooked). He used this to cut a tiny sliver off the bars of his cell door. He then sliced open the door (breaking the sliver) and killed the guards, taking their weapons. Now of course he's screwed, because alarms are going off calling for more guards.