He has pressured me into sex more than 600 times, literally how can I stop him?

[mAidrianam]

I have been going out with a guy for two years and eight months. We are both 15, turning 16 next month, and we have been having sex for two years. Every time I see him, he wants to to have sex. Whether I see him everyday, every other day or once a week, he wants to have sex each time. If I say no, he will ask until I get annoyed and let him. Sometimes he doesn't even ask, he just starts to pull down my pants. If I stop him, he will ask constantly and say please constantly until I let him. He is nice to me and treats me like a girlfriend when I first see him, then after we have sex, he treats me like a friend. The sex usually isn't even good, he usually just pulls down my pants to about my knees and does it. Don't say it doesn't work that way, because it obviously does. How can I stop him? I try so hard to say no, but he keeps asking and it annoys me. Or he says something that upsets me and I let him. If I say no, he gets upset and doesn't really talk to me for the rest of the night. How can I stop him from doing this? I am really sick of it. We've had "sex" around 600 times (literally, since I use to see him every or so for two years) and I have wanted to do it maybe 10 times. Thanks.