"Are you two hitting each other again? Typical".
Absolutely typical, especially in boys. I won't go into all of the societal issues associated with marketing to our sons and daughters. How boys get one set of influences and girls get another. No, here we can only deal with those aspects a parent can affect in and around the home and via relatives and friends.
First off, and most obvious, is that if your child sees such behaviour in the home between his or her parents, then there may be a greater likely-hood that the child will display similar behaviour. Look to yourself first and recognize that fighting, whether physical or not, is hurting those around you.
Second, is to simply observe your child's environment. Look outside during play time and re-examine the toys in use. Check out the friends and do a little friend proofing. You may find that a bad apple in the bunch is a very effective teacher. Hey now, that's your job.
And third, is to review the list of activities and sports your child is involved in or watches. Some sports support better value systems than others and when your child watches legalized violence on TV, what do you expect will come of it? Are they mature enough to handle the impact. Many adults can't separate aggression on TV from their own lives. Why would you expect a child to be able to. Start making rules and tell your child's aunts, uncles and grandparents what the rules are. You can't change them but you can instill a little guilt.
It is possible that one of you has even proposed or installed a method for releasing all of this aggression. That, like a punching bag, may not be a solution but rather an approval for punching and hitting. Be careful. Many old solutions can backfire. Work on positive influences at arms length, leaving the youngster out of the picture for the short term.
Once you have addressed any sources of aggressive activity, then you can turn your attention back to your child, focusing on direct positive influences like good-value sports or creative activities and exercise. By then you will understand better how your leadership is to be proactive. It's a bigger challenge than you think but you are, after all, responsible for another human being.