Fiona, I hear what you're saying about not wanting all the NRE to go to his newest relationship. NRE is a great high and I still enjoy a lot of it with my b/f. But fact of the matter is, NRE doesn't last forever. At some point your relationship will transition to the more settled kind, with or without him dating someone new...just hold on, the love will still be there
And I totally understand that it is hard to adjust to your b/f dating when it's something he's hardly ever done since you've been together. Change is never an easy thing; it may take you a bit of time to adapt and that's okay. I think if he truly loves you he'll be happy to reassure you when needed and give you the time you need to accept his new relationship. Does he still give you the attention you need and deserve? Or have things changed to much too fast?
I'm sure him mentioning the new person during your 'special time' was probably just an oversight. He's probably not trying to hurt you...but sometimes guys can be oblivious. Next time, when he starts talking about the new lover, just gently shush him and remind him with a smile that you'd rather talk about something else NOW and her, later.