since you two werent in a relationship at the time, i dont think you have any right to govern his actions with other girls. while this is true, he probably acted like a bit of an arse by getting with the other girl, and while you may be frustrated, dont take it out on him. he'll want to back off and think you're controlling him even though you dont really have anything formal going on.
just bite your tongue and get over that hurdle.
that being said if it's really tearing you up then talk to him about it, but don't get all posessive. just say it was hurtful because you thought you two still had something. ask him if he honestly feels the same way and make sure you know where you stand so you dont get into that kinda mess in the future. if he does want to be with you, then make sure he knows that to get you, you're the only girl he'll be kissing/hooking up with/etc and so on.
edit: woops. i dont know if a girl or guy started this thread. oh wells. you know what i mean.