Trust me. You are not going to "hurt him". That is non sense on rye toast. If he acts hurt he will be putting on an act that goes back since Adam and Eve. If he walks away he is a loser. You have an entire life ahead of you to find a main squeeze, and sex is only a small (albeit important) part of true love.
You're not going to hurt him by turning him down. He's probably expecting it. If he goes away, believe me he is not worth you even having worried.
It's your body and your decision. It's completely ok for you to not feel ready. If this boy truly cares about you and your feelings then he will understand and will not get mad when you tell him you're not ready. If he does get mad than you need to tell him that it's not ok for him to pressure you into doing something that makes you uncomfortable because people who truly care for each other never do that.
If he gets mad and breaks up with you over it then trust me he's not worth it. I've had a couple of bf's like him and only when I got older did I realize that boys who threaten or actually do break up with girls about THAT in particular are totally manipulative and disrespectful and therefore not a good person to have as a boyfriend anyway so let him go if that's how he is going to treat you.