Unfortunately, we do not live in a society where people grow up thinking that non-monogamy is a real option for the long term. It is very similar to how homosexual relationships were not really accepted. So people tried to force themselves to fit the social model they were taught (monogamous heterosexuality with an occasional divorce).
I have heard many stories of people becoming poly or accepting their poly nature. He may or may not accept it. A lot depends on how he sees his relationship with you. If you can approach it in a way that says that he is not losing anything from you, he may learn to accept it. Maybe try to get him to real some polyamory litature and try to get him to hold off judgement until he has read more. I don't know the success rate of this approach, but I know it has worked for some people who never thought it would work.