Never Look Away by Linwood Barclay

It starts with a trip to a local amusement park. David Harwood is hoping a carefree day at Five Mountains with their four-year-old son Ethan will help dispel his wife Jan's recent depression, black moods that have led to frightning thoughts of suicide.
Instead, a day of fun turns into a nightmare. When Jan and David become separated, David's worst fears seem to have come true. But when he goes to the police to report her missing, terrified that she's planning to take her own life, the nightmare reaches a whole new level.
Unable to prove that his wife was ever in the park at all, suddenly David's story starts to look suspicious - suspicious enough for the police to wonder if she's already dead, murdered by her husband.
To prove his innocence and keep his son from being taken away from him, David is going to have to confront a terrifying possibility. That he's become the victim of a cold, calculating schemer prepared to destroy him and his family to keep a dreadful secret.