How long has it been since he swallowed it? Often times it takes 2 or 3 days for things to go through the digestive system. Does the child have a "poop" every day, or just every so often? I would suggest taking him to his regular doctor and see what he says. But the ER doctor seems to me to be taking a proper approach-if it had been somthing sharp, that would be a whole different story. It might not be easy to see in a BM (poop) if the BM is large, so you may try holding a (fairly strong) magnet over the BM and see if you feel any pull. If you do, the metal is in there, even if you can't see it.