We have the same problem! DS8 is all math, and says he's "not a fiction kind of guy." He likes to read math textbooks, and textbooks of any kind (just renewed a college physics book at the library, which he has to have because it's two editions newer than the identical one that we have at home), dictionaries, books on paper airplanes and origami and magic tricks and science experiments. He also (don't tell him comics are fiction) loves Calvin & Hobbes, Garfield, Heathcliff, MAD Magazine books like Spy vs Spy, and those sorts of things. That's what he's doing this summer, so far.
Luckily, the school has an excellent summer reading program in the interests of cutting down on summer regression, so he feels fairly motivated to try for the prizes. Getting him to write it all down for the program is another thing, however.
