Just wanted to mention that evacuation was DURING the war in Great Britain not before the war. But this is a minor point. I don't really know of any good books but it seems others have. There are a lot of books that refer to it like C.S. Lewis's 'The Lion the Wardrobe' and many others. The wikapedia page has a long list of books that were inspired by the evacuations. http://en.wikipedia.org/wiki/Evacuations_of_civilians_in_Britain_during_World_War_II

If you want to introduce your child to information about Turing keep in mind that the "The Imitation Game" is a very good movie but not completely historically accurate. (http://goo.gl/asMPkB) Not said to discourage you from seeing the movie just keep this in mind. I really like this comic by the same name (http://goo.gl/bQOAXQ) This is an online comic intended for teens & adults so read it yourself to see if it's appropriate to show your son as the later chapters do have to do with his homosexuality.