Luckily teens who sexually abuse can be helped. There is an insanely high chance that it will never happen again (~90%) if they follow the right guidance and take the necessary steps to keep themselves and others safe.
As Dam8610 said, he could have been abused himself, or he was struggling with his sexuality. I never read his history so I don't know the details, however I have worked with many teenagers that have lived successful, healthy lives once they went through therapy.
Yeah its horrible and inexcusable, but a teenage boy who sexually offends is not the same as an adult. If he went to therapy, is doing EVERYTHING he needs to do to keep himself and children safe, and is now living a normal life, I would give him a shot.