What is Denny's connection to the Woodcock boy in the novel, Olive, Again?

Denny was full of regrets and self doubt when he found the Woodcock boy overdosing on a park bench one night. He then called the police, who promptly resuscitated the boy. Upon leaving, the police told Denny that he had saved a life that night. The life he saved, however, was not merely the Woodcock boy's, but also his own. As he returned home, he realized that he still had many years ahead of him, giving him the rest of his life back.