I Will Find You by Harlan Coben

Five years ago, an innocent man began a life sentence for murdering his own son. Today he found out his son is still alive.
David Burroughs was once a devoted father to his three-year-old son Matthew, living a dream life just a short drive away from the working-class suburb where he and his wife, Cheryl, first fell in love--until one fateful night when David woke suddenly to discover Matthew had been murdered while David was asleep just down the hall.
Half a decade later, David’s been wrongly accused and convicted of the murder, left to serve out his time in a maximum-security prison—a fate which, grieving and wracked with guilt, David didn’t have the will to fight. The world has moved on without him. Then Cheryl’s younger sister, Rachel, makes a surprise appearance during visiting hours bearing a strange photograph. It’s a vacation shot of a bustling amusement park a friend shared with her, and in the background, just barely in frame, is a boy bearing an eerie resemblance to David’s son. Even though it can’t be, David just knows: Matthew is still alive.
David plans a harrowing escape, determined to achieve the impossible – save his son, clear his own name, and discover the real story of what happened. But with his life on the line and the FBI following his every move, can David evade capture long enough to reveal the shocking truth?