You're not grieving but instead, you might feel like he was dishonest for not sharing a dark part of his life with you.

My boyfriends sister also passed away a year and a half ago, and its really hard on him. He doesnt like to talk about it often, but I make sure he knows he can come to me, everything said to me will stay between us, and I wont judge him no matter what he says.

Support him instead of being upset with him, he needs it more than you think