My boyfriend wants to have sex but I don’t, help?

My boyfriend and I have been dating for a little over 7 months now. He is 17 and I am 15, nearly 16. Him and I are very close and I’ve known him for over a year. Sex has come up in conversation here and there, but I’ve never felt comfortable committing to it. We both are still virgins. He’s told me he’s ready, but I’m not. I feel stupid for not feeling the same way. How do I tell him I’m not ready yet without upsetting him?