Enapay,
Guilt is a huge problem for any survivor, and in a post here all I can do is offer a few tips on how to look at this problem as you will see in your boyfriend.
Maybe the biggest problem is that we tend to look back from our current perspective and ask why, at the age of 8 (for example), we didn't do something that we would do (or think we would do) now as adults. That isn't fair, of course. A child does not have the resources to work problems out like that and in many cases doesn't even know that he has choices.
Close to that one is the problem that we often fail to "connect" to what it means when we say "I was an innocent child". The memories of abuse are too terrible, and over the years it seems that we file off the sharp edges if we can. I found that looking at old photos of myself at the age of 11 was an enormous help to getting back in touch with the pre-teen I was when my abuse started and dumping the huge burden of guilt that I carried for so long.
You may also find your boyfriend talking about what he "knows" about himself, and behaving very differently five minutes later. This is part of the big problem of knowing, rationally and intellectually, that we could not have been guilty of anything, as opposed to actually believing this. The two feelings are very different.
It's great that you are trying to help your boyfriend, but could I also say here that you should watch out for the trap of trying, as the loving girlfriend, to take all this burden on yourself and solve or fix it for him. It doesn't work that way. He has to proceed at his own pace, and many times that is just not in his control. Meantime, you do have to take care of yourself and not let your own feelings and priorities get vetoed all the time by his.
I mention this last point because I wonder if your boyfriend has really worked through his issues as completely as he thinks. The issue of the girl may just be the only thing he can talk about so far.
Here is why I wonder that. If he has dealt with the issue of his own feelings, then he has accepted that he was just a child, had no responsibility for what was happening, had no choices from which he could have selected, and was not emotionally capable of saying no or saving anyone, not even himself. If he is cool about all that, then he would also know he bears no responsibility for not intervening to help the girl. The fact that the one issue is still a live wire suggests that the other one is as well - that one and maybe many others.
Much love,
Larry