I hope to be in the same boat as you soon. Things are going well with current BF, and we MAY be taking a trip this October. He has never been.
What makes WDW special for me is 2 things. The first is the memories from my very first visit as an 8 yo child. Everytime I've been back it's like reliving my childhood!
The second is all of my childhood Disney memories, like the old Disney cartoons, Davey Crockett, Wonderful World of Disney on ABC, the books, movies and everything else that kept me entertained as a kid. I see and feel all of that, and Walt's presence, whenever I go. This is what makes it special for me!
That, I think, is the key to making a believer out of someone who has never been. They need to conect to the park using their own memories of non-park related Disney from their own childhood. Try to find out what (if anything) your fellow's fav memories, movies, characters, etc. are, and try to focus on things related to his own childhood. That is the best way for him to become a believer, IMHO.
Congrats on you upcomming wedding, and good luck!