You can't insist on burial, and in fact there is no legal definition of 'next of kin'.
You can appeal to the parents to change their minds and have a burial but they don't have to.
I they insist on cremation, you can help your son by making sure he understands that his Dad is in heaven already and as a Christian I am sure he is aware of the body as the shell and not the person.
Try and tell him factually that his Dad is not being buried but is being cremated. Tell him the process of the service and what happens while he is there. If he asks 'technical' questions about the cremation process you will have to tell him honestly otherwise his imagination will take over.
Perhaps your Vicar could be a support at this time.
This is a nice little booklet you can download:
http://www.efbox.co.uk/kids-explain-death.htm
Make sure your son understands he won't be watching the cremation - children do tend to get ideas lodged in their heads. He will be at a nice service to say good bye to his Dad, and the cremation will take place afterwards, when he is at home.