Hmm, well lets see. My first kiss was in the front seat of a bus ride to school when I was in 1st, no...2nd grade. My smooth self threw the adorable little redhead that pounced on me and gave me a slip of the tongue into the bus window. I thought I was being attacked.

I had no real interest, I was 7 maybe? I DID however show interest in 4th grade, as that was the first time I hit on a girl. It was a vastly unpleasant experience, and the girl provided me with my fear of knee-high boots that day. She was 8 at the time, and damn near to fully developed. Aside from about 3 feet of added height, she doesn't look much different today than 14 years ago. That, and being taken in by a varsity cheerleader when I was 5 and hearing her conversations for 20 minutes every morning before school probably had a lot to do with my interest so early.
I must note however, after that, I never really bothered with going after girls until I was 16. To be more specific, I never bothered with girls until one went after me and she was persistant enough to make me think it might be worth the time and effort to add new stress to my already far beyond hectic life. I am, and have never been considered a "normal boy" though. Most of the guys in my school were trying to get girls when I was 13 or so. It's a prestige thing, you see older guys doing it, and you want to do it to be cool. That, and the girls start to develop and act differently, not to mention dress differently. I never attempted to fit in, so the only time I dated in school was as a means to prove guys were being idiots. Which was a completely immature and idiotic thing of me to do in and of itself I must add.
I feel obliged to add also that you may want to talk to your son and discuss with him the pro's and con's on dating while young and in school. It's a good learning experience, however...the effects of the opposite sex on a teens mind and school experience can be staggering. He will need to be aware of how to balance his time and all if he does start dating. I feel obliged to add that I didn't listen to any advice given to me when that first pretty girl was all over me and tried using me for money. I got screwed over, and learned my lesson the hard way. If your son doesn't listen, *shrugs* chances are he will learn a valuable lesson from it eventually.
@ diggitydan, Just because a person finds themself attracted to a member of the opposite sex, or happens to be dating one, doesn't mean they're sexually active. Hell, I dated 5 girls at once in high school. I didn't bother with sex until after I graduated though.
@ DW, One other thing I forgot to add. I don't think you'll have to worry about the sex thing for at least a few years. From my experiences, it's the girls that end up having sex young more often than boys. I have known far more girls to have ended up becoming sexually active between the ages of 12-15 than boys who started between the ages of 14-16. Even if he does start dating soon, you probably have a good 3-4 years before worrying about that in all seriousness. Still, it wouldn't hurt to discuss it with him, as you never know. Some people develop young, like my old friend at 8 years old.