Did you talk to the travelling merchant (on the path to the cursed abbey), then to Reginald (outside Tom's house, just west of the SW corner of the castle walls) and tell him you would kill the trolls to avenge his son's death? Homer does not offer the troll quest after that.
If that is not the case, you should be able to ask Homer about his farm, and then get a dialog option about the trolls.
