what happened to the boy at the end of the story?

Over dinner, the boy and his grandmother discuss the next steps to take in their plan to destroy all witches. The grandmother says that there must be another Grand High Witch by now, which upsets the boy; he feels as if his being turned into a mouse was for nothing if a new Grand High Witch can still keep control over all the other witches from other countries. However, the grandmother reveals to him that as soon as they got home from England she called the Chief of Police in Bournemouth and pretended to be the Chief of Police of Norway and got the Grand High Witch's address from the hotel. They realize that they can make plans to sneak in there, find out about all the other witches in the world, and take them all down one by one by turning them all into mice and releasing cats to catch them. The story ends with the grandmother and the boy anticipating much work and excitement ahead of them.