I've thought about your question, and tried to compose an answer based on my experiences at the time as well as based on seeing how my abuse affected me later in life. Before formulating a response, it's important to realize that this is a difficult thing to do, reveal the secret. The boy is telling you this because he trusts you and believes in you. Your response has huge implications in the healing process. Your words can set the boy on the right path of healing, or, if the initial answers are off base, can lead to long term damage. Please respect the faith the boy has placed in you and be compassionate in your response. The initial response needs to be affirming that the boy did the right thing in telling. This is a confusing time for the boy. He is confused about what happened. Often times the abuser intimidates the boy into keeping it secret, saying things like other adults don't understand or that bad things will happen if he tells. Therefore, it's a sign that the boy truly trusts that adult to whom he reveals the secret. That's why it is so important to say that he did the right thing in telling. It's also necessary to tell the boy that he is safe. No one will hurt him, nor his family, despite what the abuser may have said. Other comments which are important for the boy to hear are that he is a good boy, and the fact that this happened doesn't mean he's bad. It's not his fault that this happened. He didn't do anything bad or he didn't cause it. It's also important to tell the boy that what happened isn't an act of love. His perp was not trying to show him that he loved him. It's important that you don't say he was trying to hurt him either. It's a mistake to say that the perp is a bad man. Often the perp is an adult the boys loves, respects or admires. It's very confusing to hear that someone you love is bad, because that reaffirms the belief that the boy is bad. After all, if you love a bad guy, that means that you are bad too. It's not necessary to say that the perp will be punished, only that the abuse will stop and the boy will be safe.
Along with the content of your responses, it's important that your actions along with your words support the boy. Don't interrogate him. Refrain from asking a lot of pressing questions. The boy is wrestling with a lot of demons at this time. He needs to be safe and protected, and coming at him strong with lots of questions will undermine the belief he's safe. Just say things like - " It's good to see you" or something like that. Just be patient and supportive. At times, saying nothing is the best thing.
Regardless of how supportive you may be, please remember that long term consequences will likely occur. The boy may withdraw from society and lose interest in things he previously enjoyed. His schoolwork may suffer. He may have trouble getting along with others. All of these are common results of CSA. Its important to remain patient. Sometimes, survivors don't like to be singled out, like being called on to give an answer. Anything which results in all eyes being on the boy can be devastating.
I guess, it's important to realize that not every victim will respond the same. The best thing to do is reiterate that the boy is safe and it's the end of the abuse.