After dedicating two decades to combating crime and violence in the South Bronx, retired NYPD Lt. Eric Dym made the decision to sell his house and embark on an envisioned thrilling journey with his family – traveling through Asia with the intention of learning jiu jitsu, immersing in Buddhism, and reveling in the natural world.

However, instead of fulfilling their dream, Dym and his wife Luiza were thrust into a distressing ordeal when their 7-year-old son Derek was diagnosed with a rare and aggressive form of brain cancer.

Over the course of the last year, the financially struggling couple and their two children have been residing in a recreational vehicle located near the premises of the Los Angeles Children’s Hospital, where Derek has been enduring a battery of arduous tests and treatments.

“We have no idea how this happened – he’s a healthy kid one day, and his life changed in a second,” said Dym, 45, an ex-Marine and co-host of a podcast on policing.

“We’re not giving up. We refuse. We have to stand up and fight against cancer.”

The family was in Bali when Derek began to suffer headaches. At first, a local clinic brushed it off as nothing serious. Sensing something wrong, the Dyms brought their son to the hospital where a CT scan confirmed a mass.

The couple, who also have a 3-year-old daughter, Ellie, left all their belongings behind and headed to Johns Hopkins Hospital in Baltimore. But they stopped in LA, the first US city, to rush Derek to an emergency room. After the family spent a month in a hotel, then three months as guests of the Ronald McDonald House, Dym bought the RV, and a truck to pull it, to make their home.

The Dyms have cashed out their 401Ks and maxed out their credit cards to make sure Derek gets anything he needs in a desperate bid to save his life.

His inoperable tumor, located beneath a vital area of the brain, hardly budged with radiation and oral chemotherapy.

After a clinical trial rejected Derek because the mass had grown, the Dyms paid $5,000 for a 30-day supply of medication they hoped would slow its progression. Insurance finally covered another drug costing $40,000 a month: “It was a struggle to get approved,” Dym said.

Dym gets an NYPD pension and city health insurance, but $50,000 in co-pays and out-of-network medical costs piled up. “We have to start from scratch all over again,” he said of finding the funds for future expenses.

The anguished mom and dad dove into research of DMG Glioma, a frequently fatal cancer which strikes about 300 children a year in the US. They consulted doctors in every major children’s hospital, and read every study – all with a frustrating lack of answers.

“They don’t have a cure, and they don’t know how it forms. They can’t tell us how he got it,” Eric said.

“I want to know why this is happening to my son, and how to fix it,” Luiza added. “It’s MRI after MRI, prayer after prayer.”

Hoping alternative treatments might help, she bought a $4,000 red-light therapy helmet and a $10,000 sound-therapy machine, but their impact on the tumor is unknown.

With a shunt in his brain to drain fluid, Derek was able to play with other kids and live happily for five months. He enjoyed Minecraft, a video game he received on a toy shopping spree from the Make-a-Wish Foundation.

But recently his condition has worsened. Today, the 8-year-old can barely walk, and his right arm is immobile. Fatigued, he sleeps most of the day. Little sister Ellie holds his hand and helps feed him.

Yet Derek faces his ordeal with amazing courage and a thumbs-up, his dad said: “This kid’s my hero. He’s brave. He never complains — and that gives me strength.”

As word of Derek’s plight spread on social media, the Dyms have been overwhelmed by support, and grateful for a flood of donations meant to ease their financial burden.

A  GoFundMe page launched by friends this week has raised more than $154,000 to date.