Huh. I don't know if this is what your bf is experiencing but when I'm listening to my friends' stories and they share something really personal I tend to get very emotional as well. Part of it is the shock value but a lot of it has to do with the fact that my friends are very close to my heart and the idea that they have been hurt in such terrible ways is kind of overwhelming. Because, well, I want to be there to help my friends when they are hurting (or even prevent it) and that I couldn't help them in any way. Though depending on the friend I hold it in, if I can tell they don't really want me to be emotional about it.

How long has it been since you told him? Perhaps giving him a little time to settle with the idea will make him less emotional about it?