I (having never been in your position) would tell you to stay quiet!

While you think there may still be something between you two, he is committed to someone else now. If that should end in the future, having nothing to do with you, then you can express your feelings for him at that time.

But in the mean time, you dont want to be the one responsible for breaking up two people apparently bound for the altar.

I know that this advice, like most advice when it comes to matters of the heart is more easily said than done, so I wish you the best of luck!

JBean