Wilbur will be in a cave with no light and hear voices around him. Go back up the rope and ask Ivo for help.
As Ivo, talk to the two-headed ogre and you'll get a Clicker. Take the Clicker to Bill's crane outside the school entrance. Here you'll be able to capture starlight with the Clicker. Return to Wilbur and lower the Clicker to him.
As Wilbur, return to the cave and use the Clicker. The room will fill with starlight.