A six-year-old Perth boy will be given chemotherapy for a brain tumour despite his parents' pleas to not let him become "a lab rat".

A doctor intervened after his parents refused treatment for their son, who was diagnosed with medulloblastoma in December.

His parents wanted to give their son palliative care rather than go through the "toxic hell" of chemotherapy.

The boy's mother said she had watched her mother and stepmother die from cancer, despite undergoing radiation therapy.

"The children are not really alive, they are completely drugged and exhausted and on the verge of death," she said.

The boy, who had surgery last year, could die within months without treatment, doctors said.

He was given a 30 percent chance of surviving for five years with chemotherapy, and a 50 percent chance with both chemotherapy and radiotherapy.

The boy will start the first round of treatment shortly after his upcoming birthday party.