Try to get the boy's father to find another school that has a more understanding approach to a child who has just lost his mother.
Am I right that his mother spoke English with him?
If so the child is refusing to read and write Spanish in order to feel closer to his mother. He doesn't realise this, but the adults around him need to.
If they continue to live in South America, he will learn to read and write Spanish, sooner or later, but pushing him now will only make his resistance to the language worse and probably result in other problems, like bed-wetting, stammering, you name it.
Is there any grief counselling available for the children? Probably their father would benefit from it too.
I am dreadfully sorry for your loss, all of you, was the child's mother your daughter or daughter-in-law? Please let us know, how the family progresses.