Well, not that I want to become part of the receiving end of this abuse, but I say go for it. My son is active (plays lacrosse, hockey, runs a little, bikes a little), and he was dead set on running the River to River Relay last weekend. Not that I don't have confidence in him, but FOR GOD'S SAKE, he's 10, so I told him I doubted if he could finish all 3 legs (there are some decent hills and his legs totalled 10+ miles, including 2 miles of constant uphill). As a compromise, we signed him up, and agreed that he would run what he could and I would be an alternate runner and finish up what he couldn't run. Well, after the first leg (3.6 miles at an 8:45 pace), he said, "Mom, if you want to run at all, you should run with me, because I'm running all three of these legs!"
And he did. Racked up some roadkill too.
On the way home he said he wanted to run a half marathon. I called my local race organizer and they agreed to give him an age exception to run the half marathon (their normal age min is 13, but St Jude Memphis Half Marathon has an 11 year old min). He's going to raise money for St Jude in honor of one of his friends who received cancer treatment there, and run the half.
Just because some kids can't doesn't mean other kids won't.
Keep him in good shoes and on a good training program, and he'll be fine. My son is going to train 3 days a week and be a heathy kid (hanging out at the ice rink or swimming pool) the other days.