I think other people have covered most of your questions very well.
It sounds like you are very worried he will have the "I am poly" talk with you and how you will respond if he does. If so, ask him how he feel and why. See if there are compromises. For example, maybe you could feel ok with an online romance? Or maybe he would just be ok going to strip clubs with you? Or maybe he is interested in swinging or threesomes?
Then you need to see how important it is and what each of you is willing to give to it. Maybe you allow for a little and see how you feel? Or maybe you just say you can't handle it.
It is never easy dealing with. One thing I have seen is that people do change over time. Stuff you may think you would never like; you may enjoy, and things you enjoy; you may end up hating. I don't think there should be any permanent rules in a relationship because people change over time. Let it stay in negotiations so the relationship can reshape as people's desires change.