Give him time. Some kids get obsessions, some don't. Unless it's causing problems in that he has rituals around play or he won't play something else if cars aren't available, I wouldn't worry.
From age 2-4, ds was obsessed with garbage trucks. I have a picture of him near my hospital bed the day after dd was born, pretending it was a garbage truck
. No matter what he was given, after a bit, it became a garbage truck.
From ages 4-5, it was fire trucks
. For his 6th birthday, all he got were lego fire trucks.
From 5-7, it was buses. I can't tell you the number of buses we rode, the number of 'routes' he outlined.
From 8-9 we had a 'break' from obsessions. Now we're back to them and the current ones are: Penguins and Baseball. (Which are, of course, combined and the penguins play baseball in our living room. We have bases currently taped to the carpet.
With each obsession, ds has played out things that interest, intrigue or scare him. I can't tell you the number of things that we've learned. For a while, ds knew every single bus line in the city. I swear his obsessions with buses taught him to read, because he would read the route signs. It also taught him a lot about geography. His current obsession with baseball (penguins are a sideline) is helping him explore issues about probability, statistics, fair play, popularity, performing under pressure, making mistakes and probably more that I simply haven't noticed yet.