LGL,

What a scary time to be going through with your SO. These two links are probably the best starting places for finding out more about DID and dissociative disorders. Good luck with this and please make sure to seek support for yourself as well. It is important to recognize that your efforts to assist him can take a large toll on you. Do what you can for him but make sure that you're taking care of you first.

The International Society for the Study of Dissociation info page

Sidran Bookshelf Catalogue by Subject on that page are links to more information about the disorder and treatment