There are some things you can do mostly by just shifting your perspective that can help tremendously. For starters, I have seen a lot of women get caught up on this issue and as a result, they bring it up more and more, smothering every ounce of joy from the relationship. At that point the relationship stops being fun and full of happiness and starts becoming more like a battle of wills. My recommendation is to avoid badgering him about the topic. Bringing it up once is enough, trust me.
So what makes a guy want to call you his girlfriend? Well, the most important advice I can give is to have a high sense of self worth- know what you want and know that you CAN get what you want and never have to settle for anything less. I would say focus on being fun and lighthearted and enjoying the relationship for exactly what it is.
But beyond that, I would especially recommend that you keep your options open and you continue keeping up with all your other social activities that do not involve him: It is important that you keep your time and company only as available as he is willing to be — match your level of commitment to his so that you are not left hanging. Now, some things to think about… If you like the relationship for exactly what and where it is, then stay.
If you are not satisfied with it, exactly as it is, then you should keep looking. There are a multitude of reasons why he may not call you his girlfriend. Maybe he just got out of a relationship like in this case and does not want another commitment that ends in pain. Is He Losing Interest? I think there are a few traps in this sort of situation.
If it has not been working for you for a while, then your best bet is probably to lay your cards out on the table: One woman who is pretty much happy in her relationship goes out with her girlfriends. Still they are her friends and she trusts their opinion. It reminds me of my overweight PhysEd teacher in middle-school.
All in all, relationships take work. But at the same time, it is a sobering reality to see a relationship for exactly what it is and not what it could be. Only you know if the relationship is working for you or not.