He's not over you yet. If not what would posess him to call you just to tell you that he had a girlfriend? He's trying to get your attention, be it negative attention or positive. Oh, and his new girl, totally a rebound. Why would he ask out a girl the next day after he met her...we did that in high school. Now, we date each other to see if we're really compatible for a relationship, without jumping into it right from the start, just to say we have boyfriends or girlfriends.
If I was in your position, I wouldn't let it bother you. You've broken up, it's the past, now focus on your new man and the future.
lyk