I think he’s feeding you a line of what my grandpa used to call “malarkey,” or what is commonly referred to as “B.S.” How can you have a committed, exclusive relationship that is also casual? Casual and exclusive contradict each other, like “jumbo” and “shrimp” or “Glenn Beck” and “calm, evenhanded political commentary.” A casual relationship usually means that he can see other people.
Sounds like he wants to be with you, but also leave the door open. By telling you he wants a “committed, yet casual” relationship, he has a fail-safe for when you catch him with someone else. “But I said we were casual.” Mind games: the board game men have been playing for centuries.
Chances are, he doesn’t know what he wants. Protect yourself: get him to define what he means by “casual.” Does he mean that you’ll be exclusive, not date anyone else, but it will be casual in terms of long-term commitment? Sort of a “let’s see how this goes” type of thing? Or casual in the sense of, “I can screw around behind your back but I’ll get jealous and possessive if you do.”
You need to clarify what he means here. It could just be that he wants to be with you and only you, but keep it light to start. (No talk of moving in together or other long-term plans.) But I am confused by his wording, which strikes me as a tad head game-y. And unless we are talking head games of the rockin’ Foreigner variety, you should be wary.