Roberts, who raised Neon and his twin sister Electra on organic foods, wanted alternative or less invasive orthodox treatments.
Her request was refused and she fled with Neon, sparking a nationwide police hunt. The NHS gained court backing and Neon started six weeks of x-ray radiation and chemotherapy on January 10.
Roberts said parents should decide what was best for their children.
"I wanted Neon to have proton beam therapy, which is a less aggressive form of radiation used in America, Switzerland, France and China. The NHS funds travel and accommodation for children like Neon to be treated overseas but I was never informed. This [was] never about health, quality of life or safety, but about money," she said.
Roberts has vowed to keep fighting. "With legal aid denied, I have set up an emergency appeal to raise urgent funds. It is now not the cancer he is fighting, but the painful and life threatening side effects," she said.
She said she visited Neon in hospital and remained on good terms with her ex-husband Ben.
"Ben and I stand together to support our son," she said.
Sally Roberts' appeal fund