Give him a drop of blood so he can be free and you can then loot his chest and finish this quest.
If you ask him Who drained him, then follow up by telling him he can have your Source, he will take 1 source point and then leave the area, asking you to meet him later. You can walk away and let him live, or fight him. He killed a caravan to absorb their source. After crossing the Troll Bridge, you'll soon spot the skeleton.
Any other response will result in him getting angry and leaving. The one strapped to the table can help you if you freed him, and he will also get free during battle if he takes any damage. If you start hitting the buttons, or linger too long, other evil skeletons appear and battle begins.
