What do I say?

If you have decided to confess, then personally all you can do is throw yourself on the mercy of the court, so to speak. Don't make excuses for what you did because he won't want to hear them. Just be honest and tell him you messed up and more than anything you want his forgiveness. Tell him you wanted him to know even though you realize it might hurt him because you want honesty in the relationship and that you want to do whatever it takes to regain his trust. DO NOT promise to make it up to him. You cannot do so and promising that may lead down a path that will not help your relationship.

Keep in mind that it will take time, assuming he believes he can move forward with you, and that is no sure thing. If he forgives, he will need to suppress the urge to throw this back in your face whenever you two have an argument. To help mend things, it might be good to talk to a counselor of some sort. (If the two of you regularly attend a church, maybe the minister could hold a few sessions with the two of you. That would be the least expensive way to go.)

Thank you. I do hope he can forgive me, because I would be completely lost without him in my life. I just hope he sees how sorry I am for what I have done.
As for the counselling, He is not a fan of discussing emotion with strangers, so that may not happen. but I will suggest it. I will even tell him of this and other forums, where he can find support.
I just worry what this will do to our families, as both of us have become strongly tied to the others family. He spends time with my family when I am not home, and vise-versa. I hope we can make it through this, I really do.

I just read your other thread. I am completely with ehBeth on her questions and points to you.

Since you are strongly in the direction of honesty about what I think is none of the fellow's business given the circumstance (but I get that is arguable), then he should also be honest. I am doubting that is going on.

In the meantime, your youth and learning about people is zipping on by.