I don't think there's any negative points. I've been using the net and forums since I was 10, I had my first domain and website at 11. I'm now 18 and can probably know 10 times as much about the internet as my family and friends, and am making a reasonable income. I've gained a lot of experience and I'm still young. I'd say it's a good idea
Yaaa, Sure that a good idea, But make sure you can supervise & say some safety measure require for successful running.
I live online, and work a good 10 hours a day on the net - computers have been in my life from a young age, I remember playing 'Prince of Persia' back on my 286 with a yellow and black screen... I also enjoy reading and being out doors... the balance is key for a kid. Make sure he gets out, plays with the neighbors and kicks a ball around too. I saw a high PR blog a couple of months back that someone had started for his kid to practice his creative writing, with time the kids writing got better , traffic and PR picked up, and now they guy is monetizing the blog and putting money towards a college fund for the kid.
get him on blogger so you don't have to put much time into it. Let him expiriment and see if it is just a phase. Once he posts a few times he may tire of it...