I will find you / Harlan Coben.

David and Cheryl Burroughs are living the dream - married, a beautiful house in the suburbs, a three year old son named Matthew - when tragedy strikes one night in the worst possible way. David awakes to find himself covered in blood, but not his own - his son's. And while he knows he did not murder his son, the overwhelming evidence against him puts him behind bars indefinitely. Five years into his imprisonment, Cheryl's sister arrives - and drops a bombshell. She's come with a photograph that a friend took on vacation at a theme park. The boy in the background seems familiar - and even though David realises it can't be, he knows it is. It's Matthew, and he's still alive.