OP, I think your boyfriend has been crucified enough in the comments. Yes, he probably should have phrased it better, but you know what he did? He effectively ended a conversation at a WEDDING that would steal the spotlight from the bride and groom.

I think it is incredibly selfish of you to have expected him to "scream from the rooftops" (another commenters words that you said you wanted that reaction). You can want that on any other day, but NOT while at someone else's wedding.

If you're going to discuss this with your boyfriend I suggest working on a scripted responses if the two of you don't want to announce it until it is finalized. I also suggest you consider that he's not the only one at fault here.