Gullible boyfriend targeted by delusional mother?

My boyfriend's mother is delusional, and feeds my boyfriend delusional thoughts. For example, the other day she told him that he was immaculately conceived. She's also told him that she believes he is Christ incarnate. Among many other far-fetched delusions, she's also very paranoid, anti-government, anti-doctors, believes in aliens, believes in all of the end-of-world prophecies, and suffers from what I believe to be undiagnosed mental disorders. My boyfriend is very gullible. I don't think he's delusional like she is, but believes what she tells him. He was home schooled by this woman (who never actually gave him a real education), so he thinks she's full of wisdom and "truth". It drives me crazy. Every time I try to tell him that these ideas are psychotic in nature, he gets really hurt. How do I safely address this situation?