J.H.
Sounds to me like he is asking for attention. He gets it when he hits, even if it is negative attention he is still getting what he wants. I would talk to him about how he feels when people are mean to him and explain that if he is feeling aggressive then he should let you know and together you can make a plan. I know this sounds crazy, but it worked well with my daughter. She now lets me now that "I am having a hard day" and she goes into her room and screams into her pillow and kicks the mattress a bit. Then when she comes out I pay extra attention to her. I ask what she wants to do and we play one on one for a little while. She is much better with her brother now that we do this.
Good luck!