Pleurisy. Most likely, it is a lung infection which is causing inflammation in the lining of the lungs (the pleura). Take him to go to the doctor ASAP (the pain can get worse if you let it go--I mean, like really bad--I've been there). His symptoms are pretty classic for pleurisy: pain with cough or deep breath, pressure makes it feel better.

Insist on a chest x-ray and/or a chest CT. Do not leave without pain medication and ask specifically for anti-inflammatories. It could be various types of infection, including mild pneumonia. I had horrible pain before the pneumonia even showed up on the chest x-ray. Don't worry--my doctor said lung cancer doesn't present this way (that's what my mother worried about since I had a history of thyroid cancer).

The first (useless) doctor I went to thought mine was my gallbladder (despite having a cough and shortness of breath and absolutely no other gallbladder symptoms). Although since his is on the left and he's so young, that probably won't happen to you. They still may want to do an ultrasound of his abdomen to check for kidney stones.

The good news is that once the infection clears up, the pain should go away. The anti-inflammatories really helped me in the mean time. Also, ice the area and buy one of those big ace bandages and wrap his body where the pain is. That should help.

Please write back if you have any more questions.