What could happen if my son gets hit in the sternum?

Probably nothing. However, there is a condition called commotio cordis, where the sternum is struck and it can cause sudden cardiac death. This involves the electrical system of the heart, and if the strike occurs in the right part of the cardiac cycle, it can cause ventricular fibrillation, which is fatal if not defibrillated immediately. This is why chest protectors are recommended for many sports.