Convince him he needs a dentist and afterwards you can use your tooth detractor on him. K'Chunk's tooth is just pickpocketing. And how do I convince him? The chat options just make him mad. Talk about the pain, he'll say something like: "You make it worse stop" and keep using other phrases to enhance his pain. After a while you'll get a bar (just like Gnarly's happybar) where you'll need to select the right (friendly) phrases to convince him in letting you pull his teeth.