I would tell him that his father's almost certainly going to be fine, but that risk is a part of being a soldier, just as it is in daily life. You can tell him that it's riskier to be an elevator repairperson than a soldier (from memory I believe this to be true), and that his dad is a very good soldier who will do his best to be as safe as he can be.
Then, personally, I'd let him watch the news with me, since I'd want to encourage his interest in world affairs. I certainly don't fault you for not wanting to do it, though. I just think you've got to play to a child's strengths, and that kids tend to be able to cope with more than adults give them credit for. Treating the risk as something not to be blown out of proportion, though real, might actually help him relax; so might knowing more about the details of the situation in Afghanistan.
One last point: the more he appreciates the risks his dad takes and why he takes them, the more he will deepen in his apprecation of his dad's heroism. That's the sort of thing you want him to model in his own behavior, and there's no reason he can't start mulling such stuff over at a young age. So much the better, in my opinion.
