One question ...how is he managing to smear poo all over a car without the babysitter noticing, pulling over and putting a stop to it? How much attention can he be getting...she's only a few inches away from him. I don't think you need to look so deeply for the answers, but I do think your right that this is attention seeking...from boredom...and clearly working if even auntie is this worried about it. I think its the babysitter you need to look at not the child

Change baby sitters. He probably doesn't like her. And she sounds inadequate in responding so slowly.

Such behavior expresses emotions. He shouldn't be punished but try to find out what the emotion is and resolve the cause.