The son of a legendary NYC firefighter is marking Father’s Day by hauling more than 16,000 pounds of World Trade Center steel across America, a rolling monument to honor his hero dad, who ran into the hell of 9/11 and never came back.

Stephen Siller Jr. is paying homage to his father, Stephen, who ran two miles through the Brooklyn-Battery Tunnel, lugging 80 pounds of gear, to the South Tower. Siller was never found in the 1.6 million tons of steel and concrete rubble.

His son, who was just 9 months old when his dad was killed on Sept. 11, 2001, is now driving across the country with a 16,900-pound, 21-foot-plus steel beam recovered from the South Tower in tow.

“My father’s body was never recovered, so who he is ingrained in that steel,” said Siller Jr., 25.

Siller Jr. is part of the Steel Across America tour, which began in May and is traveling a total of 10,500 miles, stopping at national monuments, museums, memorials and MLB baseball stadiums in more than 35 cities in 21 states — with the WTC beam on a flatbed truck.

“We’re sharing those stories of heroes, men like my father. And those stories are going to inspire the next generation yet again,” Siller Jr. told The Post.

The tour will culminate at Ground Zero on Sept. 11 to mark the 25th anniversary of the terror attacks, a day when Siller Jr. and his family will retrace the heroic last steps of his father, who served with Squad 1 in Park Slope, Brooklyn.

The 35-year-old Staten Island father of five was off duty that day and was driving to play golf with his brothers when he heard the call come over his radio.

“So he turned his car around, went into his firehouse. When he got there, he realized his unit had already responded. So he grabbed his gear, threw it in his truck, and started to drive to the World Trade Center. But they had every entry way into Manhattan shut down. You could not get in unless you were an emergency vehicle,” his son explained.

“So he parked his car outside the Brooklyn Battery Tunnel” and ran to the burning towers.

Siller Jr., who lives on Staten Island, works full-time for the Tunnel to Towers Foundation, the nonprofit his uncle Frank Siller created in his dad’s honor. The foundations has doled out millions to families of Sept. 11 first responders, military veterans and other heroes.

It has built dozens of homes for disabled vets, and paid off the mortgages of widows of first responders killed in the line of duty.

Tunnel to Towers is hosting the Steel Across America tour, whose stops have so far included the Flight 93 National Memorial in Shanksville, PA, and the Emma E. Booker Elementary School in Sarasota, Fla., where President George W. Bush was reading to students when he received the news of the attacks.

“We had Andy Card there, who was the man who told Bush; he was his chief of staff. To be there with him and to see his reaction to that day. It was overwhelming,” Siller Jr. said.

“We also had the teacher that was in the classroom. So Andy came down and re-met with the teacher for the first time since September 11th. It was a heck of a reunion.”

Siller Jr. recalled one of the most powerful moments of the tour.

“At our first stop, we had an FDNY firefighter whose whole unit was killed on September 11th. He was off duty that day. Everybody else in this house lost their lives. And he came to that steel, walked up to it. He started to break down. He was hugging it and crying. His friends . . . they’re a part of that steel.

“It was one of the most beautiful things I’ve ever seen . . . hugging the steel like he was hugging his friends.”