When Joshua was 2 he would not play with toys cars (hot wheels and matchbox) like his brothers, he would try and try to stack them. If they fell he would get upset and if he couldn't get them to stack he would become very upset and grab anyone to help him stack those cars.
Oh yes, stacking too. I liked to line up my stuffed animals into rows, or do the same with blocks. I was especially interested in watching spinning things -- anything that spins. I would endlessly spin the handles to my drawers, I acquired my own collection of tops, and I would get my parents to spin coins for me (as I was not coordinated enough to do it myself).
When he's interested in something it becomes an obsession
Yes, that's very much typical. And it can be a good thing....my obsessive interests have enabled me to do an incredible about of detailed research I think most people would get bored of. That can be of great help in getting a career. But at the same time, it can be a detriment. My interests cycle through one by one, and it can be very challenging to focus on what I am "supposed" to be interested in if I happen to be in the phase where I am interested in something else entirely. Your son might have trouble focusing on schoolwork for the same reason, if he is like me. The challenge, I think, is to help him find something at school he can really excel in and stick with it. This may include learning how to manage time such that he can tolerate devoting time to things he is not interested in while still indulging his interests when he is done with schoolwork.