My 7 year old son often complains of chest pains. What symptom is that?

Many. There are many causes of chest pain and to help sort this out one needs to give a good history and description of the pain and a careful physical exam by the physician. Afterwards additional tests may be required to determine the cause. Causes can be musculoskeletal, lung, cardiac, or gastrointestinal in origin. Examples would include chest wall trauma, asthma and heartburn to name a few.
Most likely heartbrn. If located in the middle above the stomach & worse when pressing in over this area it likely represents"heartburn" or the backwash of acid from the stomach sometimes called reflux or gerd.Occasionally kids can get rib cage pain in the broad distribution of the breastbone.Pain over the side or back of the chest can be a sign of irritation of the lungs from a pneumonia. Your dr can help sort it out.