First, show that you are reluctant to part with your boyfriend, and express your desire to be recognized by his family in a way that his mother is reluctant to part with you. Your dream proves that his mother shouldn't hate you in reality, otherwise she won't express her desire to be retained to retain you.
Second, you are a caring girl, and you take care of your boyfriend like a mother in the process of getting along with him, so saying goodbye to his mother in the dream symbolizes the end of your "mother" role, that is, the end of your similar "mother-child" relationship with him. (This is purely a personal guess)