How to ask boyfriend??

🐛Nicole🍦
Hey! 
(Short version: how do I respectfully ask my boyfriend about his brother who died a few months before we met?)
Full version: So I've been with my boyfriend for 5 months now. I knew since before we started dating that he had a brother who passed away, but aside from the fact that he existed, I know nothing of what happened. His dad's girlfriend treats me like I know what happened to his brother, & I just nod along like I do, but I really know nothing.
One of the first conversations we had (on Tinder but still...) was about him being upset because it would have been his brothers birthday, and I comforted him, but never pressed for more. Now we're in a committed relationship for half a year and I still don't know how to ask him without being insensitive. I just really want to know about his brother because I know he was a big part of his life but I'm afraid to ask.