Princess Milk ~Junyuu Ero Icha Seikatsu♪ Bonyuu Igai Nomenai Karada ni!?~

Masaki Katakura is an ordinary student with nothing special about him.
One day, he’s summoned to a royal palace in another world, where Princess Monica asks him to save her kingdom by… having a child with her.
Monica explains that by bearing the next generation, she’ll gain immense magical power—enough to bring peace to the land.
To make things more complicated, the food in this world doesn’t agree with Masaki’s body. The only thing that can sustain him is Monica’s breast milk, which carries the royal family’s special qualities!
And so begins Masaki’s sweet new life—unlimited access to the gentle, pure Princess Monica’s milk, and a whole lot of baby-making.

[Translated from dlsite]