A few weeks ago, my daughter had some rich home-made mac and cheese and threw up from bouncing around afterward. So, it's possible that theirs is rich enough that the car ride makes it come back up.
I also agree with the advice to call the manager at a non-peak time to see if they can help you identify the ingredient that bothers your son. My son has food allergies and when we've had unexpected reactions, I've found managers to be extremely helpful to me when I've asked about ingredients. My son does react to certain types of butters and margarines and it took us awhile to pin point that, but managers showing me the ingredients and brand of their tub of butter sure helped us out! If your gut says it's more than rich food being too much for the car ride, I'd ask them some more questions so you can find commonalities if this happens again in a different scenario.