If he lied to you "a couple of times" and they were lies that he knew would bother you, then chances are very low that he'll change, even if he means it. Change your relationship so it doesn't need trust in the things he lied about, if you're fine with such a relationship. Or you can leave HIM to clean up the mess he caused: Let HIM think of a way how he can regain your trust. My guess is that his promise that he'll change isn't new either, so unless he's VERY convincing, don't believe him.
Sleep with his best friend. That way if you ever find out he has lied to you again, you can lay it on him. He'll lose you and his best friend.. Winner!