Hmm, that makes it a little harder, having already been through the trauma of chemotherapy...because the chances of survival are so high, I have to say that he should continue the therapy. If the cancer were to return, then he should be able to decide what path he wishes to take.
Has the child been psychologically evaluated to know whether he is capable of understanding the decision he has chosen? It may help to know how he feels, without the presence of his parents.
We can't say for sure if natural therapies help to cure cancer, despite the fact there have been no studies on the subject. Chemotherapy has been shown to be the best option here, and so anyone wishing to begin natural therapy has that option, they can do so while having chemotherapy at the same time.
I think it depends on the individual. The child may fully understand the consequences of his decision, but because of his age, it is automatically assumed that he does not have the mental capacity to make such a decision. A psychological evaluation is needed her, I feel, as this may give us an idea as to whether this child fully understands the ramifications behind his choice.