Found: Thriller 1980s or 1990s

Looking for a thriller I read in the 1990s (published 1980s or 1990s). The main character is a boy, probably 10–12 years old, who has made a home living secretly inside a city zoo. Early in the book, first chapter I think, a seemingly sweet older couple — like friendly grandparents — approach him, but he quickly realizes they're predators who want to harm him. He tries to get away by claiming he needs to use the bathroom, but the man insists on watching. I don't remember how he escapes, but I believe that he does. The boy remains the protagonist throughout, I'm pretty sure. It was a dark suspense/thriller, probably by a popular author. American city setting.

I was reading a lot of thriller authors at this time. I had thought the book was by Dean Koontz, but it seems it is not. I don't think it's Stephen King or John Saul either. It could be one of the investigation thrillers by someone similar to John Sandford, Jonathan Kellerman, or John Grisham. It could be a female author like Mary Higgins Clark or Iris Johansen. I'm fairly certain it's a thriller, not full-on horror.

I found it! It's Billy Straight by Jonathan Kellerman.

Touchstone: Billy Straight