Reference from the Holy book..
The Noble Qur'an which was revealed to Prophet Muhammad(pbuh) about 1400 years ago mentions how the sex of a human being is determined. The following two verses or Ayat of the Qur'an are very clear on this matter.(A. Yusuf Ali translation)
"And of him He made
Two sexes, male and female." Surah Qiyamat (the Resurrection), 75: 39
In the above Ayah, " of him He made two sexes, male and female" means that of the human male (he), He (Allah)
Created both the sexes male and female. Hence human male is responsible for the birth of a male child or a female child. The human female is not responsible for the sex determination of the child.
Tell him to stop doing adultery and looking for a babyboy, he has to marry and stay with his wife, because doing adultery(zinaa) will not benefit him. Islam spoke about sex determination 1400 years ago and only male is responsible for sex of a new baby.