I am so sorry that your son is having such a hard time. I am sure this is taking its toll on you and your family. I can't imagine how scared for him you are. I hear you say that there has not been anything in his life to trigger the new behavior. Can you be sure that something outside the home has not occured, perhaps at school, or a friends house, is he in sports or another after school activity where something could have happened? Is it possible that he met trouble over the internet? Is there a family history of depression, anxiety or other mental illness (maby a grandparent, aunt or uncle)? Sometimes an undiagnosed learning disability or ADHD can trigger behavior problems.
Let him know and show him that he can trust you and confide in you. He is suffering, although you cannot condone his behavior, try not to make him feel like he is a bad kid, he is obviously dealing with some serious emotional issues. If you have not already, I would research mental illness on the internet, type in his symptoms and sometimes you will find books on amazon which apply. Be careful there is alot of scary false info on the internet, use reputable sites such as government sites, college sites and mental health associations and foundations. Keep searching for the right professionals. Make sure that you and your son feel comfortable with the professionals that you are working with. You may need to see 10 different professionals before you find the right fit. If you happen to read a book or find a web site that you can relate to and you agree with their philosopy, contact the author (I have done this). Spend time with him doing things that he likes and at these times do not discuss his problems, take him out for ice cream and talk about other things and if he does not want to talk, then just sit there. Keep this up, he will eventually start talking. I know that going out for icecream is not the happy fix it all, but I think that you need to open your line of communication and rebuild trust. From what I have read about cutting, The person is so numb with pain that they cut to release the pain and to feel something.
Take care of yourself, I am sure that you have so many mixed emotions: anger, fear, frustration, love, hate, all these feelings are o.k. and normal. The stress must be horrible. Keep in mind that you are doing everything that you can and that you WILL find help for your son.