Going Balistic When We Get Out Of The Car

We have a 6 year old male Mini Aussie. He is a wonderful dog except for one thing. When he is in the car with us he goes ballistic when one or both of us get out of the car. We have walked away from his sight and watched him. He just lays down and goes to sleep. He doesn't destroy anything, he does not do it if he is left home alone--just in the car. We have tried giving him a treat or a Kong. He eats the stuff but only after he has this big fit. He looks vicious when he does this and scares passers by. Help!