The boy, who is being treated at Freedom Fields Hospital in Plymouth, has leukaemia. He has received chemotherapy but is now very weak, and may need a blood transfusion. Witnesses regard blood as sacred and believe that the Bible forbids transfusions.
But Mr Justice Thorpe said that the child's interests came first. At Bristol High Court yesterday he ruled that the child - identified only as 'S' - be treated with blood or blood products at the discretion of doctors.
The judge described the boy's father as 'a young man both impressive in his emotional control and the sincerity and simplicity with which he states his convictions. His acceptance of the inevitabilty of life and death and his faith seemed to make it easy for him to conclude that faith is not to be abandoned because it leads to awful decisions.'
However, the judge said that 'either the consultant had the authority to treat S, with the discretion to administer blood, or there was no medical treatment that held any prospect of a cure'.
Michael Passmore - a Jehovah's Witness and chairman of the local hospital liaison committee, which aims to promote better understanding between Witnesses and doctors - said that he was appalled by the decision.
'In Roman times, Christian parents would sacrifice themselves and their children for their beliefs and their principles. Parents who are prepared to sacrifice a child in wartime are hailed as heroes, and yet if they sacrifice a child because of a religious principle, they are condemned.
'It is against God's law for us to tamper with blood. What if he (the child) contracts HIV in later life? What if he gets hepatitis? This violates the word of the Bible.'
But Margaret Edwards, manager of children's services for Plymouth Health Authority, said that the treatment would give the child a fighting chance. 'All we wanted was the ability to carry out blood transfusions if we need to. His condition may deteriorate at any time and we must have the power to help him.'
Last night, the boy was said to be 'comfortable' in hospital after being prepared for a transfusion.
The case is the second in a week in which Jehovah's Witnesses, doctors and the judiciary have clashed. Last week, the Court of Appeal ruled that a young woman could be given blood, against the wishes of her mother, a devout Witness.