I suggest you start working with a family counselor. You can't expect to 'solve' this issue overnight and you're at a loss. You need to work with someone who has expertise in defiant/oppositional behaviors. that person can coach you on what you should do and how to react. They can also work with both you and your DS on the family dynamic.
I don't have experience with this group and teenagers (I have teenagers but worked with them long ago) but I'm confident they can either help you or point you in the right direction.
http://www.insteppc.com/
If you want more recommendations, I suggest you post in the Kids with SN forum. There are probably a lot more people there with experience in this area. Good luck!