///I cannot see that anyone can make the decision to end the life a anyone with fully functioning mental powers.///
A child does not have fully functioning mental powers - hence the reason that a lot of decisions have to be made for them (eg smoking, drinking, driving etc)
As I said, I do not know a lot about this case or illness, but the mother is quoted as saying that her child is in 'intolerable pain'. If the child is perfectly happy and not at all distressed, then I would be prepared to concede that a mercy killing would not be right, but if he is in constant pain, then I am not convinced that putting him out of his misery would be better for all concerned.