Honey, I am so sorry you are going through this. And although you think you were willing because you knew what sex was, your really too young to make an intelligent, well informed decision on this and he knew it.
It is called statutory rape, and yes, you should tell your parents and what happens happens. Not only are you left with a "child" to take care of all on your own, he goes off and probably has sex with another young girl (child really, sorry) and gets away with this again. I know you think your parents will be angry and so be it, if they love you when they really think about this, they will realize that HE was the more responsible party he was 18 and you are underage. HE SHOULD BE LOCKED UP. Made responsible so it DOESN"T HAPPEN AGAIN. So sorry you got caught in that situation. I will pray for you and hope that you have a good life. take care sweety.