He is right now in Earth teaching kids at Rye and also attending North Salem High as a little turkish boy.
The use of this narnia name came from scroTUM, and aNUS, hence referring to your chode as Mr. Tumnus. Giving that special area of your body a name just makes it a little more personal. Kinda like naming your junk.
(Recent reports and sighting of him suggest he is attending high schools posing as a student)
Girl (lucy): What do you mean?
Mr Tumnus: Get the .... in my house I'm gonna stuff you full of food and ..... on your face.