How to say NO to sex?!?

I just turned 15 and me and my boyfriend have been going out for 4 months. The other night me and him where hanging out. We where making out and things where awesome but then he started undoing my belt! He said "please baby we have been going out forever!". I told him to stop and that I wasn't going to be his little whore. I know that I'm to young. I'm not religious about it I would just rather wait till I'm out of high school. I get that all guys have sex on thier mind all the time but I wish he could just be happy with what we are. I talk to him over text but I don't think he gets it. What else should I do because it is really bothering me.