You tell him he is a jerk, and to leave the girl alone. Tell him she is your girlfriend.
You should try not to let it bother you, because if people know it doesn't bother you they'll stop. if that doesnt work tell a trust adult.
no, maybe he's just trying to get over you. Then later, he'll ask to be just friends. Don't worry, though. He'll come when he's ready.
All girlfriends are a tease sometimes. For example I'm not very good at tennis or basketball, and when my girlfriend beats me she usually teases me about it and this can make me feel very embarrassed...
Give him a 1-2 in the face.