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You could try the murderous maths books. They are zany English books for about his age. The emphasis is on entertainment with maths thrown in. I (as an adult) learnt a few things from the them so there should be something in it for him.
The English culture of maths is different enough from the American that there will be stuff he hasn't come across.
Also, the (English) Numberphile youtube channel has some interesting stuff on it - definitely not aligned to school but just interesting e.g. the enigma machine