It sounds like you answered him calmly, and truthfully. I think that's great.

Remember too that if you think of more to say or a different way to say it, you can add to what you tell him later, after more thought.

Remember there is a lot of fear, apprehension about this which probably was stirred up in the lesson at school. Let him know he can come to you with any questions and concerns he has. You can approach the subject too, by saying from time to time that's it's been a while since you talked about growth and development and you're wondering if he has any additional thoughts about it all. Even if he doesn't, this will let him know the door is always open for talking about it.

I wonder if there are books or other resources available from the school or library that you and he could look at together.

Make it exciting, wonderous, something to explore and celebrate!