Located a safe distance from the edge of the Forbidden Forest and in clear sight of Hogwarts castle and the Groundskeeper's hut is a mossy covered stump. While you may have passed it many times already this term unnoticed, Thereos has elected to use this location for his "office" for the remainder of the term. Simply sound the nearby farm bell to alert Thereos that you wish to speak with him and he will appear.

If he feels like it.