If he believes his tumor hasn't grown because his mother swears up and down it hasn't and he further believes the drs. and the courts are outright lying when they claim it has then separating him from his mother and trying to talk sense into him isn't going to work.
The court said this boy believes that the chemo will kill him. He also dose not believe he is dangerously ill or that his cancer is going to kill him.
Asking him his choice when he's brain washed and deluded is pointless. Explaining how things are, laying out the facts so to speak, is positively pointless if the child you're talking to was convinced by his nutty mother and her guru magic man that you're a liar.