How can I not be so awkward around guys all the time (and stop looking so sad)?

Aww he likes you! You don't have to smile all the time, just try to smile at people more (especially him) when they talk to you. The more you smile the more naturally it comes. When he asks you to run an order out, shoot him a warm smile, and say something like "no problem :D" or "yes I will, and you're welcome! :D" How old are you btw? If you're still in high school, its probably not a good idea to get involved with someone that much older, but if you're an adult, go for it girl! And you know what? I bet you are pretty and just cant see it, most of us women can't see our own beauty. Good luck Hun!