Apart from the Fight with his friend is he usually worse at home . Are you Mom or Dad?If the Mental Health people have evaluated him and said he is alright have you observed any other Issues in his life at school or work?What triggers his out bursts, Have you asked him to tell you if he has a problem and can you help him,he sounds as if he is sad and needs some one to speak to about his feelings
I recently discovered that there is such a thing as Bipolar II. It is a rollercoaster and very often is anger/rage. I understand your issue and it is a shame that it is so difficult to get him help.
Is there ever an instance where it would be possible to the police/work etc....send him to the hospital as a psychological patient based on your input??