If I recommend it, you're going to hate me if your son has nightmares! :)
I would take my 12 yo to see it. I personally think that the most terrifying part of the film is the refusal to show images of 911. I think that would give your kid more nightmares. It's funny because my partner has just discovered Garrison Keiler's Prairie Home Companion and we were talking about how radio makes you pay attention and actively use your imagination. So, to see how this played out on your psyche while watching F9/11 was pretty wild.
But, to answer: the footage isn't worse than what your son's seen. If memory serves, they showed the following:
A man screaming at the camera, holding up his dead toddler and placing her in a truck to be buried. not bloody or anything.
A small child's leg that had been badly charred and was wide open with a wound so you could see bone, grisled burn flesh, and arteries. It was the first time I've ever seen such an image though I've read descriptions. My friend, Judith, couldn't bear to watch it. Still, it isn't anything that is worse than the stills.
The shiny burnt faces of two women who'd been badly burnt.
A child with a bandaged head wound, sobbing with pain as they clean the wound. No blood, just the horror of watching the pain.
There's a scene of soldiers with bad wounds. The wound isn't horrifying as much as the screaming and panic.
The scene of Iraqis beating the burnt bodies of the contractors may be more disturbing. They also showed the contractors' bodies hanging from the bridge. They are incredibly charred and grisly. What's going to be terrifying to a kid is the rage of the Iraqis as they beat the bodies with a stick, not so much the charred bodies.
The images of limbless soldiers might trouble him. Nothing gruesome--no blood, guts, gore--just saddening to see them with no legs and arms.
Still, my experience with kids is that they absorb what they can handle and reject the rest. If he has a tendency to have problems dealing with this kind of stuff, keep him home. Otherwise, I don't think the film is that grisly. Frankly, as my friend J said, she found it harder to endure the trailer for some summer terror flick about a couple stranded in the ocean fending off attacking sharks. There's something about the way film imagery and music work together to make something with no blood at all feel far more terrifying than the site of charred limbs.
Kelley
"We're in a fucking stagmire."
--Little Carmine, 'The Sopranos'