Watashi no Musuko ga Isekai Tensei Shitappoi

Tomota Dobara (age 35), a nerdy office worker who loves light novels, encounters Mio Hayama, his high school classmate, for the first time in 17 years. She tells him she lost her son in a car accident but asks Dobara to think of a way to meet him, because in her words,"My son seems to have been reincarnated into another world."

(Source: Shogakukan, translated)