I lied to my online boyfriend about my age but I'm afraid that once I tell him the truth he will stop talking to me?

I've been talking to an 18 year guy online for 2 years. The problem is that when we first started talking I told him I was a year older than I really am, I'm 15 years old now. One time when we were on the phone I joked about it by asking him what he would do if i was actually older or younger. He simply said that it would be crazy, then I asked "what if I was just a year younger, like 15?" and he said that it would still be wrong. But when I think about it I don't know how being 16 would make it right either. Because he's turning 19 in a month from today. I really want to tell him the truth, but I'm afraid that he will stop talking to me now that I've become so used to talking to him everyday.