Okay, so we're dealing with two kids here. So I'm sure I'll be downvoted into obscurity by the morality police but here it goes...

Your boyfriend's issues with sex and his own sexuality stem from a great many things. His asexuality probably being one of the biggest variables. Being asexual in a sexualized relationship is like being on the swim team and not knowing how to swim.

His experiences with his brother are obviously a factor in his current sexuality, just like any sexual experience would play a part. But it's important to remember they were both kids during this time. His older brother wasn't an adult, or even a young adult. His brother was in a stage where he was instinctually motivated to experiment and explore his sexuality, and kids regularly do that with siblings or friends.

At the same time your boyfriend was in a stage where he didn't understand sex, and that was probably due to his asexual nature. Something which his brother wouldn't have known or even understood.

So I wouldn't blame his brother because it's likely his brother has very little to do with the issue you're both currently having. A lot of people have a natural anger response when their SO admits to an adolescent sexual experience with an older sibling. We like to all jump on the "you were abused" and "they're a monster" bandwagon because it's an easy go to explanation for what's going on, and we all like to have someone to blame.

I'd start trying to solve this issue by asking why your boyfriend is trying to be sexual when he's asexual in the first place. It's not a nice road to travel down and it's probably full of shame and embarrassment because that's typically how asexual people react to sexual situations. But it's that road you need to go down first before pointing fingers at other people.