Some questions and food for thought:
How long have you two been together? How long have you been discussing marriage? Is anything official yet? How old are you both?
How well do you communicate about your deeper feelings? Has he explained what poly means to him and how he would want to see it work, especially after you two marry? Would you want other partners as well, and is he okay with that?
Have you discussed boundaries such as safer sex practices and how he will manage his time? What are deal-breakers for you? It isn't poly without full consent of everyone involved, so before moving forward into a polyamorous situation with him, you need to sit down and come to some agreement on how things will be handled.
Here is a page with some checklists: http://openingup.net/resources/free-...om-opening-up/
Polyamory really works best when people are willing to talk, talk, talk, and then talk some more.
What do you hope to gain from being in a relationship with a poly man? You could wind up with some lovely friendships with his other girlfriends -- are you open to that possibility?