He may lash out at you and refuse any professional help, but from how you have described his condition, you can't just "hope for the best." Find out all you can from the parents of his friends, from school personnel, and from his friends, who may be very worried about him as well. It's also a time where siblings need to tell the truth about what they have seen and heard, even at the temporary cost of his feeling "betrayed" by them.
This is a time when all extended family and friends need to pull together to diffuse any impending violence and to form a support team, along with professional help, to help your teen regain his sense of well-being and self control.