being a stepmom and pregnant

I absolutely love my SO's son. We have such an amazing relationship that I wouldn't trade for the world. He's such a bright,smart,funny, and handsome little man. I know he's going to be an awesome big brother. He doesn't know yet he is going to be a big brother and we don't want to tell him until we know the gender of the baby. If you're a step parent when would you tell your stepchild and how? He's turning 7 in June so I'm sure he will understand what's going on.