The Forgotten Child by Donald Levin
Newly retired from the Ferndale Police Department, Martin Preuss passes his days quietly with his beloved son Toby. When a friend asks him to look for a boy who disappeared forty years ago, the former investigator gradually becomes consumed with finding the forgotten child. Meanwhile, ex-colleague Janey Cahill persuades him to help her locate the missing father of a troubled young girl. Juggling both cases, Preuss revisits the countercultural fervor of Detroit in the 1970s--and plunges into hidden worlds of guilty secrets and dark crimes that won't stay buried.
The Forgotten Child is the fourthbook in the Martin Preuss series.