I guess my opinion is "What's the Point?"
Is it your intention to get to know this guy better? If he intrigues you beyond all reason, than perhaps this is a good plan -- perhaps you WANT him to come on your vacation, because you're dying to get to know your niece's boyfriend better, REGARDLESS of what DN thinks of him 6 months , 6 years, or 6 decades from now! You're ready to invest in some extra points, and offer them at a discount to him...that extra effort, and the risk of possibly eating that extra room is ok with you should he bail out on you, because after all, you just REALLY WANT to spend vacation time with this guy?
But somehow... I don't suspect that's your plan.
Rather, if the point of this trip was to spend quality time with family -- and that was then extended to include your NIECE -- well, this fellow will only be a detractor to that cause, I would suspect. She is, in effect, saying, you are not enough for me -- I need HIM to enjoy the trip. And there ya have it. If they are even closer by then, you will only see less of her, and if they have spats and issues, well, you will be there for that and the mood swings that will accompany it, too. Ugh.
If she is paying HER way, and he is offering to pay HIS way -- they should go THEIR own way and have their OWN vacation! What really then is the point of them joining you on yours, other than to cause some extra logistics work for you?
Tell niece you're glad she has a nice BF that seems so keen on Disney. Tell her this trip is open for her, and you're looking forward to a great time together like the "old days" when you traveled together. And tell her that perhaps while you're in WDW, she can make plans for a future trip with BF. Tell her you also understand, however, if she is so excited at the prospect of a trip with this new beau that she wants to save her $$ for that. You would be happy to share your WDW knowledge with her and help guide her to the right resources to have a fabulous time together, exploring WDW on their own.
If she bails (either at the get-go or in the final hour) you aren't out anything (you get a bit more space in your room, more bathroom time, and whatever she paid for is all hers to worry about.) If she comes -- well, as you said, she's family, she's always going to be your niece, and you obviously care deeply for her, and the odds of a good time are in your favor, even if she is pining for her BF some of the time. You may have to lay ground rules, but at least, with family, an aunt, uncle, and grandparent can try that -- kinda harder to impose rules on a stranger, like Mr. Excited About Disney BF.
As always -- JMHO, which combined with a buck or so will get you a slushie at 7-11 and that's about all. Good Luck!