Moving to fast?

I’m 17, and I’m dating a 19 year old. We’ve been dating for around six months. Pretty much since we started dating, he’s talked about marriage and how he wants to be with me forever. Problem is, this is my first relationship and he really freaks me out when he talks like that. I’m scared if I tell him to back off that he’ll become so embarrassed he might do something rash. How can I gently tell him I want to slow down? I’m scared he’s going to propose at my graduation.