Well I'll see if I can think of some things that cost nothing at all:
You could head over to the Boardwalk Resort for a nice stroll along the boardwalk under a clear night sky. Some people rent bicycles for two and ride along the Boardwalk as well. Just enjoy each other's company, hold hands, chat and laugh.
At your own resort even, a nice stroll along the paths that circle the lake under a nice moonlit sky can be quite romantic, with a few sitting areas along the way to sit and have a little snuggle. Its a very dark and quiet stoll at night, with most people either in their rooms or at the pool.
If you're at the Magic Kingdom, take the monorail over to the Polynesian. They have a white-sand beach with hamocks...complete with the Cinderella Castle in the background. Very quiet, peaceful and romantic. Just lay down and swing while holding in each other's arms....can't get much better than that.
Are these along the lines of what you're looking for? If so, I'll try to come up with more...some may not be completely free, but I'll try to keep cost in mind.
P.S. If he still thinks Disney is just for kids, take him on Mission Space, Tower of Terror and Rockin' Roller Coaster and he'll soon discover just how much for "kids" Disney really is.