In my opinion the communication starts early and is ongoing. It is not a one time chat.
Believe me, by 9 he knows a lot more than you think he does. I've been in the classroom 13 years and I've overheard conversations and picked up the illustrations they have made on paper.....that the students have about sex, developing, etc. The fact that he hasn't asked you means nothing.... maybe he feels like he can't ask you. Be aware though, at 9 and younger, the children know a LOT more than you think they do.
Our school nurse comes in to talk with boys and girls separately in the 4th grade and it is shocking that some parents have never spoken to their children and this is the only info that is factual that the children receive.
I'd much prefer to answer my child's questions and have an ongoing line of communication so that I can either give her the facts or find the facts vs her believing what is said around school and peers.
If you are uncomfortable, get in the car and go on a drive so you do not have to look at each other but please.... communicate.
If you don't know where to start, go to the library for book references, ask your pedi, but by all means, get the communication started.