Actually when we are with the people we feel most comfortable with, the less our stutter impedes us. Since stuttering is brought about sometimes by anxiety internally, there must be some part of you that is scared to be completely vulnerable with your boyfriend. Whether it's a stutter or sex or burping in front of them - love needs trust and full and total vulnerability to work, trust that he loves you for you and let the overthinking subside and pass you by as meaningless emotions. I would recommend speaking to a therapist because this isn't just about your stutter, it's about anxiousness which takes work and time to get through