I am sorry to hear that you are dealing with this problem. Unfortunately, this can happen despite what the home environment is like.
However, I think the first step is to sit down and think if there are any recent changes in your son's environment that could have triggered this. Some children will act out in this way in response to stressful events - like a death or divorce in the family, a move to a new town or school, etc.
Another thing you can do is talk to your son's principal and teachers to see how he is behaving in the classroom and try to get a better sense of his overall social functioning.
In addition to trying to sort out any potential social stressors that could have triggered this, it is probably also worthwhile to talk to your son's pediatrician
. I say this because sometimes certain psychiatric or medical conditions can have bullying as a component. For example, some children with attention deficit disorder have problems with impulse control and this might appear as aggressive behavior. If something like this is uncovered, it may be that there is an effective medical treatment for the problem that will help your son's behavior.