Perhaps the feelings of abandonment stem from somewhere else? Some unresolved issues from his past?
Nothing like poly talk to bring up everything.
It might be helpful to be sure you know what you want for now. Be honest and set some time lines that feel right for you. It sounds like this is something that needs sorting before you get married, so there is a time line for instance.
Baby step, there is no rush if you want to do it right as far as I'm concerned. Build a good foundation of respect, honesty, openness, empathy and tons of love and the rest will unfold. Make sure you sharpen those communication skills. You will need them!