You need to sit down and talk to him. Be sweet. Do not be harsh or yell. If he continues to bully, I would reccomend seeking professional help.

I think he should start seeing a child therapist. It could be really helpful. Also, they could help him if he had anything that happened to him that is causing the aggressive behaviors.

I can relate to this situation. I used to be a bully myself. At the age of 8-10 I used to be the worst son you can imagine. I stole stuff, terrorized other children, be very mean to my parents (giving them the feelings that they didn't love me while they did) and so on. One day, my father cured me. He was furious. Told me to sit the fuck down on the couch. I was so flabbergasted (because he usually asked me stuff and he always was nice and holding back). So, I did as he asked. He was silent for a while, so I said 'wtf dad'. I was 10 by the way. He told me to shut the fuck up. He explained what the problem was. He told me that him and my mother had been questioning themselves what to do about my behaviour. He said 'no more wondering what we did wrong'. He told me that they had been too soft for all these years. So he told me what would happen: If i would not immediately change my behaviour, he would search for external help. No psychologist, no therapy, I would have to go to some sort of prison thing. That's what he said, and that was it. He added that if it wouldn't change within a week, we would go to that institute to have a look.

Was never necessary. I instantly changed my behaviour because I realised that the problem was me. School went better, people started to like me and everything changed.

HOWEVER - I have to mention that my behaviour was caused by the intense need of attention. I didn't realise that if I were a nice kid, people would give me attention as well. If the problem with your son is caused by anything else, you will need to find that out first, because the solution my father applied, would probably cause the reverse effect.