Oh..today's whorey youth.
At 13-15 it really doesn't matter what you say. Just tell him you are not having a baby yet so you are finding someone else.
Be honest, tell him u have met someone else. It will be hard to say but in the long term, he will appreciate ur honesty!
This will happen frequently over the next five to ten years. Just tell him, and get on with your life.
At your age, "Let's be friends, things are just not working out for us romantically" should be sufficient.... wait at least 2 or 3 weeks before starting to date someone else.
You might hold off on the "someone else" until you resove things with this young man. I assume that their are reasons why you have lost interest, so instead of telling him you like someone else, maybe have a conversation about either his behavior or his attitude that have made you lost interest. That way, you will not only accomplish your goal of breaking up with him, but you will leave him with something to think about that may help him instead of leaving him feeling inadequate. i hope it turns out well and I hope you realize that it is not essential to always have '"someone". Waiting for the right someone will protect your heart and let you live with a little less heartbreak.
i don't like you anymore