Lego, from the games online ( Worldbuilder, Bionicle online games and stories, Junkbot , puzzles etc, at the Lego site), to the objects themselves, esp bionicle, and other technic stuff, also Playmobil.
Also drawing very complex plans and diagrams of all sorts of things, maps, worlds, mazes, processes, game steps, etc.
Reading Tintin, Schtroumph/Smurf, and Asterix comic books, amongst others. Telling himself stories from all of these sources, esp. Bionicle.
He used to be into dinosaurs, passionately, drawing pictures of them and reading books about them, but that was earlier. Robots and aliens/space travel too. But less engulfingly.