I know this is a difficult situation, but I think that under no circumstances should you intervene.
One of a survivor's greatest traumas is that of betrayed trust and a feeling that all the world is unsafe for him. If you tell ANYONE, I am sure that would be only with the best intentions. But then other people would act with similar intentions, and finally it would reverberate back to the younger brother and probably also your boyfriend. Both will feel "outed" and disempowered.
I can assure you that if this were to happen to me, it would be months before I would ever talk to anyone again. It would not be a matter of sulking, but of reverting to an extreme defense of boundaries.
Perhaps someone else has a suggestion for a clever way through this impasse, but frankly, I don't see one. The difficulty is that once you tell someone what you know, you have no further control on what happens with this information
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)