It's so great you want to help and support your little brother. That's going to be very important to him to see he has you on his side. Here are some thoughts you might find useful:
DO SAY: "I believe you". Abusers keep boys shut up by telling them that if they ever say anything no one will believe them. Telling your brother you believe him invites him to break the silence and talk to you.
DO SAY: "It wasn't your fault". Again, abusers often tell boys that they were in on it, that they liked it (because their bodies responded), and so on. And so often a boy will think it's all his fault anyway. He needs to know this is absolutely untrue. It is never the boy's fault. The blame always belongs 100% to the abuser.
DO SAY: "It's okay to cry." What has happened to your brother is a terrible thing, and as he faces it he may well become overwhelmed by his emotions and feelings and need to cry. Let him know that it's a male macho bullshit myth that "real guys don't cry".
DO SAY: "You're not alone and you can talk to me." One of the most crushing burdens of abuse is that feeling of being utterly alone. Many boys have no idea that others have been abused too. They think it's only them, which in turn reinforces their feelings of shame and guilt. Your brother needs to know that one out of every six boys in the USA is abused before he reaches his 18th birthday. He is not alone.
There are also some things you should watch out for:
DON'T SAY: "Why didn't you say no/make him stop/tell/run?" Even if you ask these questions only out of curiosity and interest, they will sound to your brother like he is being blamed. More than 90% of the time a boy is abused by someone he knows and trusts, and when it happens the first time he often freezes and cannot respond - everything is happening too quickly. Thereafter shame and guilt take over and keep him silent. Quite often the boy feels worthless and thinks he deserves the abuse. All of this is of course false.
DON'T SAY: "Don't worry. You will get over it." That shows the boy that you have no idea how catastrophically he has been harmed. He won't just "get over it"; in most cases he will need professional help. Saying "get over it" will sound to him like his feelings and pain don't count.
DON'T SAY: "I feel so sorry for you." Your brother will need your care, affection, attention and love, but if he feels he is being pitied he may resent your best efforts to support him.
You might also want to let your brother know about this site. We have a great bunch of teen guys here and they have their own forum where they can talk to each other. The site is carefully moderated and our priority is safety for everyone who comes here.
If you have further questions please feel free to post them, or PM me or one of the other moderators.
Nobody living can ever stop me
As I go walking my freedom highway.
Nobody living can make me turn back:
This land was made for you and me. (Woody Guthrie)