My son was a sorter (and an organizer...loved to line things up). When he was about your son's age, he became obsessed with matchbox cars. Once we had accumulated several, he started sorting by category (safety vehicles, speed vehicles, etc.). He never went anywhere without one in each hand and several in his pockets.
He also loved to read and sort tiny baby books (about 4" x 4"). He had two sets of different colors (each set covered the alphabet). He would sort by subject, animals, story type, and color of the covers. These were handy for taking places where we didn't want him making a lot of car noises.
