I've been having massive trouble with this mission. Stopping the bus isn't an issue, but I can't find the creeper anywhere. Where is he hiding?

Even tho you can see where he is when he radios you at 1st. That's not where he is.
Form where u start. go forward. Use nitro and ram the bus. Head back to where you started at the mondo burger. And he will wait for you there.

He (Whirlwind) can be at random spots in the level, but always near a building. He could be at the Mondo Burger where Taurus was parked at the start of the mission. Or at another joint that you can find next to the same road.
Or at the gas station to the right of the intersection where the bus is heading left. Or you could follow that road (take a right at the intersection), pass the gas station, the road will bend to the right and you'll see another gas station.
Final other location: he could be next to a school like building to the left of the road in the direction the bus is headed.