I can answer that,
If you are at the mercy of a monster for a big chunk of your life, you don't think that a policeman can save you from squat.
No one can help you, ever.
Hope is just another 4 letter word.
There was a terrible case awhile back in CA. A monster female had 5 kids-2 boys, 3 girls-and she adored the boys and hated her daughters. She hated them so much that she physically abused them, forced them into prostitution, and ultimately murdered 2 of the girls (forcing the boys to help dispose of the bodies in fires).
One of the girls who was later murdered escaped and went to the police. She told the police everything. And psycho mom shows up, explains the daughter is a pathological liar-and the girl was returned to her custody.
It is not at ALL unusual for kids to assume that no one will believe their word over an adult. So it's VERY possible Shawn calculated his chances of asking for help, being disbelieved, and being returned to a now-enraged captor who might make his captivity far, far worse , if he didn't outright kill him.