A 5-year-old boy was shot and his father killed when a gunman opened fire in a Bronx public-housing project Tuesday afternoon, police said.

Jaquan Millien, 29, had just picked up his son from school when he got into a fight with the shooter inside the Butler Houses on Webster Avenue near East 170th Street in Claremont Village at about 4:40 p.m., authorities said.

The suspect pulled a gun in a sixth-floor stairwell and blasted the child in the arm and his father in the stomach, Chief of the NYPD Housing Bureau James Secreto said.

A plainclothed housing lieutenant from Police Area 7 named Eric Dym rushed over and used his belt as a tourniquet to stop the blood gushing from the kid’s arm.

“He was very calm. I was amazed by his bravery,” Dym, a former marine, said about the child.

The suspect then fled and has not been caught, police said.

The child was rushed to St. Barnabas Hospital and is expected to survive, cops said. His father was taken to Bronx-Lebanon Hospital, where he was pronounced dead, police said.