When Greta interviewed the boy, he said the truck went past him in the direction he was going toward his home. So he was behind the truck. I didn't get the impression from him that he was intentionally following the truck...but that the truck went down the same road to the boy's home. It was a dead end, so the truck had to turn around in a driveway and go back out.
Thanks for the details of the boy's interview with Greta. I couldn't quite hear the details.