Telling other children?

My husband and I are very excited to be expecting our first child together (my first ever) on March 5. My stepson just turned 8. He has a little sister (2.5) from his mother and her fiance. When do we tell him he's going to be a big brother again? Wait until I am really showing? (Right now I just look I've put on weight-see picture.) Thanksgiving? Christmas? This is new territory. Thanks!