Chichi no Jikan

Follows Kyuuichi Shinogi, a male biology student who lives in an apartment near a helpful female neighbor/friend, Ayaka Takarai.
One day, he is visited by aliens who demand that he raise their baby for a year… or else the world is history. The baby is apparently human,
and is raised the way humans are: through breastfeeding. Kyuuichi’s sex is not a problem for the aliens: they give him a simple device that
switches his body to a woman’s with the mere touch of a button. Conveniently, Kyuuichi’s female form is very well-endowed. Although
she’s understandably confused and uncomfortable by the whole situation, Ayaka helps Kyuuichi out as he navigates the unfamiliar
experience of raising a child – and breastfeeding it in particular.