Wait for him to pop the question and give you the ring.
(
ETA for explanation: ...since "popping the question" and having the ring are part of your story. If you were a couple who didn't think a formal engagement step necessary, I'd say go ahead and make the announcement.
You never really know what life will bring you, so maybe wait for the official question and ring before making the announcement. Everyone that I know says "oohhh...he popped the question?! Do you have a ring? Can I see it???!!!" whenever someone makes this announcement. It sucks to have to respond with "well, not yet, but during an argument...and we're getting married in December...")