I would be very careful if you do decide to continue this relationship. Abuse is carried down from generation to generation. If his mother did this, even as they were both adults, then it was likely something she had participated in as a younger woman or her brother did or something along those lines. To think that is appropriate behavior, this had to start somewhere else.

If this happened to your bf and his mother, I wonder what may happen one day when he has children. What will he encourage them to do sexually. If he sees this as appropriate then that might leave the door open for future inappropriate actions later.

Just for a reference several of my relatives would see this as appropriate behavior because they have done the same things with there mature children. One of which resulted in pregnancy. I personally have a problem with this kind of behavior.

I can't tell you not to stay with this man, I can only tell you I hear you, I understand your confusion, and I can warn you about what the future may hold. But ultimately we do not know what the future holds just be careful. Good luck in the decisions you make for the future.