Engagement announcement

What are some good ways to break the news of an engagement to my fiancé’s family?! He asked me to marry him on Mother’s Day! We haven’t told his parents yet because they tend to make a huge deal out of everything and we just decided not to. BUT now my family is about to come into town and I know my mom will mention it at some point in front of his parents. We have seen his parents a few times since the proposal (I just took my ring off so they wouldn’t see) and I’m worried they are going to be offended that we never told them those times! Help!!