I had to respond because my DS9 had the car obsession too! I would say it lasted about 1.5 years. We would buy him car catalogs at the airport to keep him occupied on flights. Walking through a parking lot with him was hilarious, he'd shout out the make and model of each car we passed.
He then moved on into dinosaurs and is currently obsessed with Minecraft and any kind of rocks and minerals. And the Periodic Table.
At that age (3.5) we were a little worried for him socially, because he did not seem to relate to other kids, but now that he is older he really seems to integrate well into groups. He will always be introverted (as are his mom and dad) and needs to decompress after school for a while, and prefers smaller groups to large ones, but he also seems to have plenty of friends at school and little difficulty adjusting to new social circumstances. It seems like he has different "sets" of friends...ie, friends he plays sports with and acts goofy with, and friends he shares deeper interests with.
