If you can get him to read a few books on opening up your relationship then that would be a start. Both of you should read together. Here's the thing though, he might simply not be willing to invest heavily in a person who is not monogamous. I think it would be very wise to explore this before you get married. You need to find out if you have compatible relationship goals - marrying a mono man will likely not make the situation easier and I am speaking from the perspective of a mono man.
Playing the Game of Life with Monopoly rules.
Monogamy might just be in my genes
Poly Events All Over