Had I been in your son's position, namely getting a meal with something that hadn't been stated in the menu, on my plate, I would have told the waiting staff, sent it back to the kitchen and not just have the offending item removed, but a whole new plate of food prepared, especially if it were coleslaw because of its sloppy nature, it could have touched the other food items and contaminated them.
I have a lot of food allergies and intolerances, so eating out is a mess about for me anyway, so I don't want to add any additional messing around having to try cutting bits of contaminated food off and pushing things to one edge of my plate, I just want to be able to eat the food I've actually ordered.
At least I don't sit pulling faces and messing with my food.