Telling step children

This is my first pregnancy but my SO has a chils who is under 10 years old. We have him every other weekend and half the summer.

There's not a whole lot of resources on how to break the news to elementary school age step children.

I suggested to SO to perhaps start by just implanting the idea in his head then after the 1st trimester actually break the news, but really what's the best way to go about it?