A seven-year-old boy is to receive radiotherapy for a cancerous brain tumour against the wishes of his mother after a high court judge ruled the mother's judgment had "gone awry" on the seriousness of his condition.

Neon Roberts, whose identity was made public after his mother, Sally, briefly disappeared with him earlier this month, will begin the treatment in the new year after Mr Justice Bodey, sitting at the family division of the high court in London, decided that doctors' concerns about the boy's health outweighed his mother's desire to seek alternative therapies.

Roberts, 37, from Brighton, East Sussex, argued that radiotherapy could impair Neon's IQ or make him infertile. Bodey said he sympathised with Roberts' position and believed she was acting out of love for her son. He said: "The mother has been through a terrible time. This sort of thing is every parent's nightmare. But I am worried that her judgment has gone awry on the question of the seriousness of the threat which Neon faces."

The belief of doctors that Neon would die within months without the treatment was paramount, Bodey said. "One cannot enjoy a quality of life if one is not alive." He refused Roberts any more time to investigate alternative treatments.

Neon first had surgery in October to remove a medulloblastoma, a fast-growing type of brain tumour that begins in the cerebellum and is more common in children than adults. When a scan earlier this month showed part of the tumour remained, his mother opposed further surgery, arguing they could be detecting scar tissue. She then went missing with her son for four days.

Neon had another operation on Wednesday after Bodey made an initial ruling against Roberts, a music producer originally from New Zealand. Early that day she applied to the appeal court to stop the operation but was again refused.

Doctors from the NHS trust treating Neon, which cannot be identified for legal reasons, recommended radiotherapy and chemotherapy to prevent the regrowth of any microscopic tumour cells remaining in the brain. While the specialists warned that, without treatment, Neon could die very quickly, his mother argued for a delay so she could look into treatments such as immunotherapy, boron neutron capture therapy, photodynamic therapy and hyperbaric oxygen therapy.

Lawyers representing the doctors told the court the experts listed by Roberts appeared to have little or no expertise in treating medulloblastoma, with one having apparently based his description of it from the internet or newspaper cuttings.

Neon's father, Ben, 34, from west London, is estranged from Roberts and supported the doctors on both the surgery and radiotherapy. Neon is staying with him during the treatment.

The father's solicitor, Gwen Williams, said her client understood the difficulties of the case but was "relieved" at the final judgment.

Williams said: "Neon's health and recovery has always been Mr Roberts' priority. While he respected Neon's mother's views, his own opinion, following in-depth discussions with doctors, has always been that Neon should have the treatment.

"He now hopes that Neon can be allowed to recover from his latest operation and start the radiotherapy and chemotherapy that the doctors have outlined without any further delay." The father has requested that Neon and the family now be left alone by the media.

Ahead of the ruling Ian Peddie QC, representing Roberts, whose case is being funded by legal aid, said she was not being obstructive and her position reflected "the love that she has" for her son.

He said: "Why does this intelligent woman believe that it is not in Neon's best interests for him to have radiotherapy? The mother, having considered the pros and cons of radiotherapy, does not believe it is in his best interests because of the significant and long-term consequences of this dramatic, powerful brain-altering treatment."

Sally Bradley, representing Neon's father, said he accepted there was no alternative. She said: "The mother is becoming increasingly implacable and that is a concern."