Sounds more like shock than calm.

is this something completely out of the blue, or have things not being going well for a while?

if he's intending to go through with it and can't be talked out of it, presuming you want to carry on with the marriage, get good advice and lean on your girlfriends that's what they are there for.

Good luck and please post more if you want. A lot of ladies here have been through all sorts of situations and can be of help.

Good luck.