May I make a suggestion for the future: Set up a nightly study time and require that your son spend that time with no music or TV. He can quietly read if he has no homework. It is amazing how much homework gets done when an adolescent has to stay there, anyway. This also makes it very difficult for him to say "I don't know." What that usually means is "I don't want to tell you" (that I enjoy all my free time hanging out or talking with my friends).
Keep setting those natural and logical consequences. You and your son will be much happier.