Well I would find out why he is doing it,has anything happened to upset him and make him act out, new sibling, how about school is there any form of bullying you know of, how does he get on with the teachers, why was he held back? Before using punishments which will make him worse ask Why, what reason does he give for the behavior? ..
This post has a lot about changing behavior. See if it helps.
http://www.medhelp.org/posts/Child-Behavior/8-year-old-temper-tantrums/show/5982#post_7210934
It also sounds like he is highly frustrated. Did all the school do was to hold him back. Did they do any kind of testing?