You should definitely talk to your fiance about it. If you are going to move in with him, you should feel comfortable enough to talk to him about something that is this important to you. I think it would be a better start for both of you if you both understood where you stood in reference to Spencer and his place in your home. You may find that it will "upset" your fiance more if you don't allow him to reunite with "his" pup. Yeah, you need to talk to each other about it.