DS really enjoyed a book called Penrose the Mathematical Cat, which is a big-picture concept book.
In talking with DS, I have always made a distinction between arithmetic and mathematics. Arithmetic--essentially the same as calculation-- is what is taught in elemenary school and focuses on tools that are helpful (like learning the multiplication tables)that we need to live our lives and what is necessary to do day-to-day calculations. Mathematics are the big picute ideas: patterns seen in nature, infinity, how codes work etc. I know plenty of people who are Mathematically gifted but despise calculation and were never very good at it; they went on to become math professors because they love the ideas.
DS learned his multiplication tables when he started having to do multiple digit multiplication and long division. Once he saw how unpleasant it would be to have to work each sub-problem from scratch, he didn't take any more convincing.
