How to break up with my boyfriend?

So me and my boyfriend have been dating for a couple of months now, but we were childhood sweethearts. He lives over 300 miles away (I know long distance relationships and such), and we are both 17. The whole reason I want to break up with him is because he's already planning on when he's proposing to me, what the wedding will be like, and has even picked out godparents for our 4 children that he wants to have, names included. I'm just not ready to go that far, after all we are just teens, and I've told him that he needs to slow down, but he won't listen. Help please.