Go ahead and reach out. Let him know that you were thinking about taking things to the next level (though, ugh to that phrase) before he told you about the other girl. You were starting to fall for him, you hoped he felt the same and you were disappointed. But since he was only doing what you asked—keeping it casual—you can’t fault him for keeping his options open, looking around, dating other girls, etc. And you can’t fault him for failing to read your mind.
Close by telling him that you’d be open to dating—a real, noncasual relationship—if things don’t work out with this other girl.