One way to bring it up is to find some polyamory news in the media. (One good spot is Polyamory in the News
.) You could use this as a launching point to see what his views on polyamory are. That may can lead into a "what if" type of situation. Like you could ask him if thinks it is possible to lobe more than a person at a time. It may show you what objections he may have to polyamory.
If it doesn't lead into a good poly discussion, you can think or research on what he has said and apporach him again. But you should be very honest about what you want and what you like. Be reassuring that it doesn't mean there is anything wrong with him. Try to explain that love is not a finite resource. Give him time to get use to the idea. I think it is good to have this take before you have a partner in mind.