My fiance wants to sing a song at our wedding...?

I agree with Blunt. This sounds ghastly from a guest standpoint. (The same applies to a song you dedicate to him, even if you don't sing it). The reason I say this, and it's what you can tell him, is that guests have a very limited span for this sort of thing. That's why slideshows should be zip-fast, toasts should be short, etc. These types of things, and especially dedicated songs, will have meaning to the 2 of you. The other 150 people there will probably be bored or zoned out.