Most Helpful Guy
Probably because he doesn't want you to do the same. Some guys like to just be called by their first name and don't go for the nicknames everyone uses.
Most Helpful Girl
I personally don’t think it is something to worry about. There are a lot of people who just don’t like pet names and feel awkward saying them. And if you aren’t in a relationship with this guy, then he really doesn’t feel obliged to call you a pet name just for the sake of it.