Anytime something is eating someone alive, professional help is called for. He'll probably find it a relief even to just be able to tell the story to someone who is non-judgemental and is equipped to help him put it into an appropriate perspective. Therapy is never a waste of time.
You must be a very good friend, as well as a girlfriend, for your boyfriend to open up to you. Since this is a lone incident, it is as AnxiousNoMore said, it is an impulsive act by two impulsive kids. Please tell him that in many many kid's lives things like this happen once and never again. He desperately needs to talk to a therapist so that they can allow him to gain this perspective and move on. In my youth, my brother made advances on me, his 2 year younger sister, and I have let it go, knowing that he was an impulsive kid, who happened to have problems, well we both did as we were being physically and mentally abused by our folks. These kinds of things happen all the time. Let him know that he is not a bad person, the kid asked him and he followed by doing what he was asked. That's a whole lot different than if he advanced himself and his wishes on the kid. He knew it was wrong and he stopped and stayed away from the boy. He did everything he could to manage the situation in the best way he could have. Please let us know that he has agreed to talk to a therapist and if you let him know you've written this story on Medhelp. let him know that we're all rooting for him to get over this and move on in a happy and healthy future.
God bless you both. You're a good girlfriend, and a good friend to have.