Why would one side of the diaphragm be elevated (and in pain)?

Could be a number of things, but it's good that his EKG was normal. If he recently performed any strenuous activity involving lifting heavy objects it could be costochondritis (an inflamed/painful area located beside breast bone); if he's coughing it could be (acute or chronic) bronchitis; there's also the chance of a tumor pushing the diaphragm upward -- but before you get all crazy, since the chest xray was (presumably) negative for a tumor or growth, it's probably not this (or pneumonia, unless the chest xray revealed some sort of infiltrate(s)). It could also be something he's had since birth (displaced diaphragm), just undiscovered due to never being sick before and having a chest xray. If he's normally a healthy individual it's probably nothing about which to worry. If the symptoms subside, he still needs to visit with his regular family doctor to let him know what happened... and his regular MD is likely to get more information out of him than an ER provider.