How do I tell him I’m not really ready for sex yet

I’ve been dating this guy for a few weeks now and everything’s great but all he wants to do is have sex but I’m not sure I’m ready, at first I was all up for it but now I’m not sure, we don’t really live that close either I just don’t know how to tell him. Another thing is he’s 18 but I’m only 15 and i have no idea what to do, he’s my cousins best mate and I don’t know how to tell him I’m not ready, does anyone have any advice or do I sound like I’m just making a big deal out of nothing