Ahh, "youth is wasted on the young." (That's an old expression that originally had nothing to do with erections, but it sure fits, doesn't it?)
It's very sweet and touching that he chose to talk to you. The fact that he was very quiet and kind of shy sounds like he was nervous about bringing it up. I agree with the mom who said that if his dad suddenly brings up the subject, he may feel betrayed and clam up. I would ask him (your son) if it's OK if Dad talks to him about it, and if he's not comfortable with that, respect his wishes and talk to him yourself. You don't have to go overboard, just... answer the questions as they come as simply as you can.
And make sure he knows that nothing is wrong with him - you don't have to explain the details about sex, but at the very least say "No, honey. There's nothing at ALL wrong with you. It means you're growing up healthy. Your body is *supposed* to do that." You didn't mention what you actually *did* say to him, so if you weren't able to get that much information out, try to bring that up yourself - you don't want to wait if he thinks something is wrong with himself.
When he brings up stuff like this, even if you're shocked, try to look cool and calm, because you don't want him to be too nervous to bring it up. Kids are smart, many of them notice that even from a very young age you talk about their eyes, ears, toes, etc. and not so much about penises, etc.
The most important thing is not to overwhelm him with too much information. You know, waiting to have "The Talk" and having it be one big serious thing. That's really awkward for everyone, and it's very easy to give them more information than they are ready for. (Not to mention that if he's embarrassed, he won't be listening very well.) So I would try to keep it light and casual, especially if you're the one bringing it up. Just deal with one little tidbit at a time, so he'll have time to think about it and make sense of it all. Don't always wait for him to ask a question, bring it up yourself now and then. Sometimes the car is an excellent place for conversations like this, because it's not as confrontational. (He can look out the window if he's not comfortable talking, or just avoid direct eye contact.)
While it's true that there are books about "Where babies come from" that are geared for many different ages, you might do best doing a little research for yourself on how to talk to kids about this kind of stuff. I think that would help more with the... casual conversations.
I have a book called "The Guide to Getting it On" which is a funny (but quite explicit) book about... all things sexual (I do mean *ALL* - I wouldn't let my kids see this book before 18, but it should be required reading by age... I dunno 20 or something.) But in that book is a chapter called "Talking to Kids about Sex" which I found very useful. See if you can find that in your bookstore and see what they have to say, but keep in mind if you're reading in the bookstore that the authors are quite open and frank you'll probably either burst out laughing or gasp, maybe both. (It's not in the parenting section, but the section with self-help, psychology, marriage and intimacy books.)
Just don't do like my mom. She was very progressive in many ways, but she still wasn't comfortable talking about this. So when I turned 12, she gave me a copy of "Our Bodies Our Selves" and it was too much information. It kinda wigged me out.