A 3-year-old boy with Down syndrome fell five stories from his Bronx building Monday — breaking his thigh bone, according to cops and police sources.

The child’s mother told investigators he pushed on a piece of cardboard next to the air conditioner in the apartment on East Tremont Avenue near Benson Street in Westchester Square around 1 p.m. Monday and fell out the window.

The child struck a store awning on the way down and landed on the ground, police sources said.

Officers found him on the ground, bleeding from his leg. He was taken to Jacobi Medical Center with a broken femur, cops said.

The city Administration for Children’s Services was notified, and no criminality is suspected.