PLEASE COMMENT BELOW! Should I announce or hold off?

My husband and I are telling his Dad on Sunday about our pregnancy! Last minute he asked if his girlfriend could join us for dinner so of course we said yes. But now I'm thinking about it maybe we should hold off on telling him? I know he'll be excited about being a first time grandpa, but he's a very privet person and I don't know where they stand in their relationship nor do I want to make him feel uncomfortable or any other way besides happy. Am I just over thinking this?