There's nothing you can say to him to truly get him to understand...but that is based on my experience over the past two years and a bit. Your best bet is to take a look at what each of you want from this relationship. Once you establish that then you can see if you meet each other's criteria to invest more.
A major part of my success and happiness in a poly relationship as a mono person is that the life experiences I needed to achieve in a monogamous relationship were already met. What are both your needs?
The following link deals with some things you might want to ask yourself before trying to explain it. You're poliness isn't likely to go away..nor is his mononess...you'll be dealing with it for as long as the relationship last quite possibly.
Survival guide for dating a mono