Hello and welcome. I had to respond to your post because it really struck chord with me; my DS9 was exactly like this when he was your DS's age. In fact, it was around 2 when he started showing us his incredible memory with cars. He also could recognize model and make of cars from lights and he could also recognize them if just the little bit of the back end of the car was visible (rest of car blocked by car parked next to it). I have to tell you, whilst he has serial obsessions to this day, this one he can't seem to get past. We had a set of approximately 30 to 40 thomas the tank engine books sent to us from the UK and he memorized the stories for the entire set when he was a preschooler. He spends of his free time creating new products for his made up companies and also draws vehicles, then draws the detail of the lights in boxes on the same page. So he is clearly obsessed with car lights! This is how we discovered he could read; by reading and remember make and models.
He also had very little interest in playing with other kids his age in preschool but he wasn't quirky enough to stand out; just always played by himself, and wasn't interested in interacting with them as much as wandering around and discovering the world. We worried about that a lot when he was younger. He is still very introverted today and usually has 1 to 2 close friends but so much more sociable, once he gets to know them, i.e. in a class setting. He is well liked by his classmates.
However, DS used to line up cars a lot and used to love watching fans. He was finally diagnosed with aspergers 2 years ago but I started questioning the diagnosis when I discovered he and his little sister are highly gifted. If he is, he is definitely on the mild end of the spectrum, but the results on the WISC IV did indicate that perhaps he does has ASD. He scored highly on the verbal subsets except for comprehension and average on processing speed. He is also highly disorganized. These are apparently results similar to ASD kids. The tester did not believe he is on the spectrum though.
I wouldn't worry too much about the social piece as you might find once he starts school he really starts to blossom in that area like my son did. And doesn't sound like your DS has any stimming behaviors.
ETA: One thing I always thought was interesting about my DS is that he never shares his intense obsessions (other than minecraft) with his friends. He told me he didn't want them to think he was weird.
