We were told he has a broken sternum, but paperwork also says closed fractures of multiple ribs with flail chest,what exactly does that mean?

Terminology: The sternum is separate from the ribs (but is attached to several).Closed fracture just means the cracked ribs stayed inside the skin.The mechanics of breathing require the ribs work in unison like parallel bucket handles lifting during inspiration/dropping to exhale."Flail" chest just means the ribs were not able to rise/drop in unison.This decreases the effectiveness of breathing efforts

Chest trauma : Has multiple broken ribs and broken sternum.