Most Helpful Guy
First, you NEED to tell him. Because you're minors, depending on the age difference and rules in your state, he could actually get in trouble. You like him. You don't want that. Fortunately, it sounds like you are less than two years apart, so it's probable that he's okay. You shouldn't be doing the hanky panky at your ages anyway.
His feelings will be hurt. He'll wonder what else he can't trust you on. But every single day you don't tell him, that's only going to get worse when he finally finds out. Also, you want to be the one to tell him, not your dad when he says, "Oh, and she had such a great fifteenth birthday a week ago."
So be as honest as possible with him. Tell him you didn't want to be rejected because of your age, that you really like him, that you lied because you like him, that you're telling the truth that you like him. Most of all, admit that you were wrong to lie, and that if he doesn't trust you now, that you understand that. Explain your reasons, but don't try to justify it or get defensive about it. Ask him to forgive you, but don't expect it to be immediate.
He'll be hurt, but he should appreciate your honesty and the fact that you understand the consequences of the lie. If he doesn't, he might need time. And if the relationship falls apart, just know that continuing the lie would have been postponing the inevitable. He'll find out sooner or later.
You're fourteen. Unless you date him for five more years and then marry him young, he's not the One anyway. So you'll feel bad and sad but you have time more than anything.
(Okay, that said, what were you doing on a dating site at 14? And how can you be "serious" at 14? Kids these days, geez. Enjoy being a young teenager, and let the relationship stuff come as it comes. Dating sites... well...)