My grandpa is racist towards my boyfriend?

I've always loved my grandparents very much but they've always lived far away in other states. I was excited when they moved in to my parents home (I just graduated highschool so I still live at home) a couple months ago. It was all great until my mom started telling me that my grandparents hated my " mixed" or half black (I guess you would call it) boy friend. I've been with him for years before they moved here and I have no intensions of breaking up with him. Ive ignored their prejudice against him for several weeks until my grandfather told me that he could not have mixed great grandchildren and he would cut me off from his family if I were to ever marry my boyfriend. He said that mixed children would never be successful and would never be accepted by society. The intensity of how much this sucks is indescribable and of course I completely disagree with him. As angry as it makes me, I know it's probably just how he was raised, and he thinks he's looking out for me. My grandparents treat me like I have some fatal flaw now and I feel like we're strangers living in the same house. What can I do???? I can barely believe this is even happening! I'm scared that defending myself or my boyfriend would make me the permanent outcast of the family. My boyfriend is the nicest guy in the world and doesn't deserve a second of the disapproval my grandparents give him!!! I haven't told him about my grandparents prejudices but I think he can feel it when he visits. Do I just have to wait this out? Please help me!!! I don't know how to handle this