Fiance giving me ultimatum

I have always wanted to have a happy family. It was my dream to be a wife and a mom. I had a son 20 years ago and my son's dad cheated on me and left me for her. I went from shit relationship to shit relationship. And then I finally found an amazing man. He's perfect in every way. I'm 40 and am currently pregnant with TWINS! This is everything I wanted. I love my fiance. He is a hard worker. He's a provider. He's my protector. He treats me like a queen. There is just one problem out of everything and it's that he doesn't get along with my 20 year old. Both me and my fiance are religious. He's just a bit more extreme than me. My son grew up in church but decided he doesn't believe in God. My son is obsessed with zodiacs and completely believes in astrology. To the point he calls me all the time to tell me my horoscope and he found out his moon sign and stuff. He has every zodiac sign tattooed down his left arm and a lion on his right arm because he's a Leo. My fiance believes astrology is evil and demonic. I am as Christian as the next woman but I honestly don't believe astrology is evil. I just think it's a diy thing we humans made up for fun. My son believes in all that and talks about the rings on Jupiter and what not and I think it's all nonsense but I let him live his life. My fiance has been telling my son forever that zodiacs are evil and demonic and my son would fight back and say maybe he thinks the idea of a sky daddy is stupid which offends me and causes us to argue but for the most part me and my son get along when my fiance isn't involved. My fiance asked me today if I loved him and I saod of course. I'm having his twins and marrying him. He said if I love him he would like me to cut off my son for good. He believes he's messing with demonic stuff and he doesn't want that in our lives are around our kids. I told him I think he's being silly. It's not that big of a deal. But he says it is and as long as he's into demonic stuff he wants him out of our lives and maybe that will encourage him to stop. He told me if I won't cut him off that's fine but we should break up and just co-parenting. He's saying these are his boundaries. I love him and he's perfect in every other way. This family is all I've ever wanted. But I don't want to lose the family I already made. But he's giving me this ultimatum.