Ok. Don't know if this is good advice or not, but just throwing it in.
Have you tried giving him what he wants? My ds has had periods of extremely fixated obsessions, one very similar to this where he all he wanted was to go outside and sit in the car, grilling anyone who visited about their cars, screaming if denied and huge tantrums when brought back. All before age of two.
What helped in the end was for me to go out and sit with him in the car for about an hour at a time, or my DH would when he was in from work. Ds would sit in front seat and pretend to 'drive', we would sit and supervise and read, Internet etc. After a few days of this he stopped asking as frequently, or would be more easily distracted. I appreciate it's difficult, we didn't have other children at the time, but he almost needed to immerse himself in it to come out the other side of the obsession.
He's 3 now and still gets fixations, to a lesser extent perhaps, but still seems to need to process them in the same way. So for instance he watched a film recently and was obsessed with watching it all the time so one day I just let him leave it on, played it four times in a loop, now he only asks for it occasionally. Before that it was all he talked about, very distressed if I tried to distract with anything else.
I have wondered about my ds too, as it seemed out of the ordinary to me, but he's calmed down with age. He does have some sensory issues too though, but is very sociable.