I have a couple of comments for you to add to all the wonderful feedback you have received. But first, I've got to restate that you are awesome to have such a proactive attitude towards supporting your brother. This stuff is so horrible that it takes courage to deal with it head on, for ALL of us.
I have been married to a CSA survivor for 20 years. My experience is with a man, therefore. I have no experience with helping a child through this. I have two teenage boys though, and they are definitely still children.
I would REALLY appreciate survivor input on my suggestions to make sure they are OK. My H has OKed them, but I know everyone's experience is different so just to be sure...
First, reading about the subject of CSA would be a helpful step, IMO. You certainly don't have to become an expert, or learn everything there is to know, but if you have enjoyed a childhood free from abuse (as I had) getting an intro into that world might be helpful. It's a cold, dark world.
Unfortunately, truth and logic often have almost no bearing on how my H thinks and feels about his abuse... and thus himself. So my use of truth and logic has negligable impact on him. We are speaking different languages.
If your brother does ever choose to share with you, it would probably be good for him if your reaction isn't shock and horror. educating yourself a bit might help with that.
Also, I think me educating myself made it a lot easier for my H to share with me. (not that the goal is for your brother to share, but the goal is that if sharing is what he wants, its available for him). When I found out that my H had been abused, I OFTEN made it clear to him that it didn't affect how I felt about him, and it was true. But the tape inside his head was playing "yeah, but you're only saying that because you dont know what I've been through, if you knew, you would feel differently".
He knew I didn't have a clue about his experience so he discounted my proclamations of love and acceptance. Once I started educating myself, that was harder for him to do. You can read everything in the world and never know what it is like to be him, true. But you can read enough to be able to say, "yeah, I know people have been forced to do X, and I KNOW it's not their fault. I know people have gone through Y and I KNOW they are still wonderful and beautiful and lovable.
Because I also know it wasn't their fault. I know you can do Z and its not your fault."
I am not saying I think you should tell him everything you are reading, but maybe him knowing THAT you are reading is a start. And then as you demonstrate understanding during conversations, he'll slowly know you really are there for him, rather than you just saying you are. this could be an extremely lengthy process (years?) and will be on his schedule. All you can do is prepare yourself not to fumble if he tosses you the ball.
Second, I think you should make sure you have support for you. This has changed your life too and you need to make sure you have help if you need it. This might just mean one or two visits to the college councellor, but if you need it, do it. It wont help your brother if part of his support system falls apart.
Third, and this is the idea that I'd REALLY like survivors to comment on because I am unsure about it. But, since you are away at college, what about writing him a letter? I know you probably talk to him on the phone and e/mail, but in a letter you could put some of the things he needs to hear (that others have posted) and he can take it out and read it whenever he needs to. For my H, reading that type of information is a lot safer and has more impact than hearing it. In letter form, it can be received as more of a gift.
Good luck awesomebro,