He knows he's just a summer thing for you, and when the summer is over you'll leave him. He knows that, but he still wants to be with you, he wants to make the most of this summer so well that you'll change your mind about him

He saw you walking with your head down, he knew you had done something with Betty. He thinks while you and Betty's relationship is strained, he wants to make you better with him

"Get in the car" he said as he stopped his car beside the street where you were walking