It's also possible that Shawn saw or knew of the murder of Devlin's previous victim, a boy who looked much like Shawn. That would ensure obedience too.
You have to be careful when you post stuff like that, so people don't misunderstand. Your post makes it seem like it has been an established fact that Devlin was responsible for the disappearance of a boy who looked like Shawn. We don't know that to be the case yet. It certainly seems possible, but you don't want to unintentionally spread misinformation.
That is what I think.