M Sensei no Koto

It's the beginning of Summer and a lady has arrived in Japan in her search for a plaything.
There's a custom in her family that allows a lady to look for a slave when she reaches a certain age, just as if she wanted a butler.
The main character is employed as a private tutor. He was chosen not only for his linguistic abilities, but also feminine looks and masochistic qualities...

From the very first day, his dick gets tortured with toys, he's forced to endure facesitting and many other things as part of his masochist training under the guise of private lessons. Eventually, he makes peace with the way things are and officially becomes a slave...

[Translated from DLsite]