What can I do to make my rabbit like my boyfriend?

My boyfriend and I got a rabbit about five months ago. I am the primary caretaker, but my boyfriend helps out. My rabbit is very loving towards me and tolerant of my boyfriend. Recently she has been really bad towards him and won't even come out of the cage for a treat if he's holding it. He tries really hard with her and she literally won't give him the time of day. We adopted her as a "family" pet, but it seems like she wants nothing to do with him and it's starting to upset him. Normally he doesn't care, but lately she's grown worse. I'm sure the main issue is that I'm the primary caregiver to her and I'm also wondering if it's because I have a naturally softer voice, so maybe that appeals to her more. Can anyone give me any tips on how to have my boyfriend approach her and gradually start to build her trust? I keep telling him to speak softly and move slowly because rabbits spook easily, and to just have patience. However, her behavior towards him isn't improving. What can we do?