The subscapularis tendon is one of four tendons of muscles that surround the shoulder. Your son may have injured his shoulder sometime in the past.

Anytime tissue is damaged, the body responds to this injury with an inflammatory response. This sometimes ends with calcification of the damaged tissue. Tiny calcium deposits develop. The resulting calcification is usually only seen with a microscope. Sometimes it's large enough to be seen on X-ray. This is more common in children whose bones are not fully mature. The body makes a big effort to repair itself with the result being an over response of calcium formation.

There are other causes of tendon calcification. Infection, tumor, and autoimmune disorders are the most common. Ask your doctor about this finding. It's likely the result of an old injury but could also be the start of a newly developing condition.