Good for you, Umb. Being brave and forcing the issue is exactly what I would have suggested. You deserve to know where you stand. It sounds like some clarity would be helpful for him too, as he seems to have confused ideas of what is or isn't a relationship. Obviously, for many people, exclusivity is NOT what defines dating, otherwise, for example, my signature line would make no sense.
Since he seems confused, I would suggest that you take the lead in being clear in the upcoming conversation. Tell him that you want a serious (another word that might fit is "primary") relationship with him (which is what it sounds like to me), which means that you live your lives together and prioritize each other and are committed to each other. But, also tell him that you're happy for him to date others, and to possibly have other serious relationships, maybe even equally serious (co-primary), and that you would like the same freedom. Tell him that this would mean that you each promise to treat your relationship with the respect it deserves, which means NOT just dumping each other if someone else comes along but instead treating the relationship as its own valuable, unique entity to be nurtured (kind of like a baby, really).
He seems to think that if he takes his relationship with you (because you do, in fact, have a relationship even if he doesn't like to think of it that way) seriously, it means he's giving up the chance to have another relationship that could fulfill his desire for a co-parent. That does not have to be the case, as evidenced by many of the stories on this board. If he find this idea confusing, you might find it helpful to read some of the introductory essays at www.morethantwo.com
and then recommend some to him.