I started at 13. I'd never take it back. Wish I'd started when I was 12 (met my first stoner friend). You can buy it, smoke it, eat it, sell it, grow it, (and get arrested for each one of them!)... it's fucking awesome. I think it should be a family thing; it is at my house. It makes my life so much more interesting. And it's a great social thing if your kid is insecure or just weird, heh.
I'm really baked now, but unless you smoked when you were his age and noticed serious negative effects, you should let him do it at least for a while (and see if it does effect him negatively, in which case you'd have him stop) imo.
My parent's philosophy was "We know we can't keep you from doing it.. may as well do it here so you dont get arrested." It's such a comfort not to have to hide things like that from your parents, and you can guide his use if your allowing it to some extent.
[EDIT] Ha, I wrote that in the sense that you were his dad, which I now know your not. But you know what I mean...