Remember Patty Hearst joining the Symbionese Liberation Army. At least Devlin didn't make the boy help him rob a bank. Did Devlin somehow persuade Shawn that he was better off living with him than with his parents?
That's a good theory. He had full control of the boy and he may have been able to manipulate his thinking on a daily basis.
I have no idea if it happened this way but from what I do know about abuse, I will speculate. You're 11 years old, snatched off the street taken to an apartment by a stranger, maybe drugged and abused repeatedly by this man. Maybe at first you fought but then relented because you are a child and not strong enough to fight. Once the fighting stops the pervert starts telling you, you can't go home now, you're gay, your parents are going to be ashamed of you and hate you. Your friends will make fun of you because everyone will find out what we did and I'll tell everyone you liked it. If you stay with me we'll keep it secret, nobody will ever know.
The fear and humiliation associated with such an attack would make a child just submit. I know of two families that were shattered by horrendous abuse. The things that the abuser told the children were so twisted, yet simple and one had to wonder why these children believed the lies. When dealing with a predator you are dealing with a sociopath who is cunning and a master manipulator, thats why they usually get away with it as long as they do.
As another poster pointed out, hopefully there weren't any other victims BEFORE Shawn. I'd be checking that propery he owns with cadaver dogs.