Wow... my son is 12..,we talk pretty openly about it. Puberty is already hitting him pretty hard. In the 3rd grade ( YES 3rd grade!), another child shared their knowledge about sex with him. I expected more of mommy and daddy kissing, maybe under the covers or in their birthday suit, but no, this child knew it all! I was pretty ticked about it, but my son wouldn't give up who it was.
In the long run, even though I still feel it was too young, it also opened the door for my son BEFORE he started changing instead of know when he is grown extremely modest about the changes happening to his body. If I had waited until there was a "sign" he wouldn't have been as accepting and more embarrassed I think.
At the end of January, he went for his 12 yr check up (turned 12 in OCt however), the dr mentioned a little hair "down there" and he has grown probably 5 inches since then, hair everywhere..even a little stache! I was a little naive thinking it would be a gradual transition! When I see him standing next to friends, I am amazed (and sad) at how much he has changed.
I encourage you to talk with him soon. My son talks to me more than his dad really about it. We are open and try to not hide our bodies. We don't run around naked, but we don't jump behind doors if accidently walked in on, but my son's modesty has done a complete turn. Which is fine and we respect that.
When we discuss sex, we discuss our views on it and how God made a man and woman's body to fit together, etc... In the 5th grade, his school showed the video. They didn't offer parents to watch it with the kids like they did when I was in school. (they offered prior to showing it to the kids, but my mom watched it with me at the same time). He came home with a few questions.
At this age, he is learning about it. HOwever, probably not what you want him to learn about it so I would definately jump in to let him know your expectations and views on sex. Also, other boys will claim they are doing it- as my son informed me that a boy told him that-I wonder and hope that little boy is fibbing. I told my son most likely he is just trying to act big. You definately don't want him to think if so and so is doing it, I should too.