I got the bus to stop by ramming it head on at full speed.
But where is Whirlwind? I can't find him.
Edit: Nevermind, I found him not too far from the start. I searched everywhere many times but he wouldn't show up. Maybe a bug, I dunno.

If I remember correctly there are about 3 places where he can be, randomized on the start of the mission. I usually just drove down the road where the bus came from till I can clearly see the first gas station, and if he's not there, just turn right and cut through the field and eventually he's there somewhere.