You've got lots of good responses here. I'll just add some of the things that I've experienced raising my three sons. You'll both be uncomfortable at first, but as with anything, you'll make it through this first discussion. Don't wait! At 11 he's been exposed to a lot of information already, and you need to be sure that the information is correct. When you speak to him, treat him with respect and listen to him (though he may not want to say anything or even look at you!)
I also want to add something that I think is very important. I think that when presenting sexual information to boys, most parents forget to discuss what girls go through. I think that it's very important for boys to know about girls menstrual cycles and how female anatomy works. Now, you may not want to discuss all of that in one sitting, but it's important that boys know about this, too. My husband grew up with only boys in the house, and he was pretty much clueless when I met him. All he knew was what he saw on Tampax commercials, I think! (In his defense, I will tell you that we were young when we met.) It's a little funny, but it is serious information that all males need. If we expect them to be informed about sex, we can't give them just half of the equation. And, it's good for them to know some basic information.
The other thing that I want to add is that you also need to discuss oral sex. This is a pretty big thing in middle school. The kids don't consider it sex, and the boys aren't refusing what the girls are offering. There has been more and more information about this reported recently, and I have a bit of personal experience with the boys that I work with. I've overhead some very shocking things, and it is very, very common. Also, my oldest son, who is 19, worked at a movie theater, and he says they had a lot of problems with the 11 and 12 year old kids who would come into the theater in big groups, and then have oral sex during the movies. From the parents perspective, they think, "Oh, fun! Isn't it nice that the kids are going to the movies all together. It's fine to go out boy-girl in groups." Well, you might want to keep an eye on that. After my son told me about it, I started hearing things on Oprah and reading about that in some magazines that movie theaters were pretty popular places to have oral sex.
It's no fun having to address these things with our children, but they need to get good, reliable information. That's our job as parents. Start talking with your son now, and be sure to keep talking for years to come! It's not THE talk, because that implies a one-time thing. This has to be an ongoing discussion.
You'll do fine!