Be stright,be kind. Explain to the person your reasons for breaking up (you should go over it first in your own head to be clear as to why),but don't offer false hope ,better to hurt now and move on then get jerked around.
Tell him you don't want to be his girlfriend. Tying yourself to a boy at your age can make you miss the life of a lifetime there are so many things in this world and a boy or man that will help you enjoy them. you will know him when you meet him.He will be the one person you are meant to be with for the rest of your life. Life is real SHORT!!