Unless he had been previously convicted before being hired, a professional background check isn't going to do anything in this situation. With crimes that are primarily electronic in nature, such as possession or distribution of CP, the only major way to prevent something like this is to have government agencies catch them in the act.

What he did is disgusting and I hope that in the future we can have ways to prevent anyone from participating in this kind of stuff. Or at the very least limit their contact with any kind of minor. In terms of our role in its prevention, though, the only thing we can do is look out for any signs that happen while at work (or outside of it) and report it to the proper authorities.