How does Chris arrange for his “disappearance” so his parents don’t know where he is until much later?

Chris actually didn't have to arrange his disappearance. He merely told his family he was going to be on the road that summer with no explanations. Taking extended road trips had been a regular activity for Chris after buying his Datsun.

".... during that graduation weekend he casually mentioned to his parents that he intended to spend the upcoming summer on the road as well. His exact words were “I think I’m going to disappear for a while.”