Well, his dad just kept telling everyone (after the beans were spilled about the money at the relatives place where they got stuck a while) that he wanted a new truck. Never had one, he said. He wanted that truck. Also there was a thing with an air compressor being stolen from him by his business partner when he lived there.
So someone finds the card and everyone in town starts making fun of him for thinking he had really won this money. When they left that town on the final leg of the journey to Lincoln, it was with everyone in town thinking he was such a loser. Then, if course, no winning ticket.
So Son drives to a car lot and trades his car in on the spot for a new truck. Not brand new, but new enough. Then goes to Sears and gets his dad a new air compressor. Then they drive back to the town and Son lets Dad drive down Main Street real nice and slow, and he hides down on the floorboard. So everyone who made fun of him are left thinking that he DID win, and there he goes with his new truck!