This could be a behavioral issue that's not being corrected by his parents, as others have suggested. He could also have some other problem with socialization that has not been diagnosed (some forms of autism inhibit social skills and conversation, ODD causes defiance, etc.). There may be something causing violent rages. It may be beyond his control and so it's not a good idea, or particularly helpful, to label him as "weird." However, that doesn't mean that you need to put up with it. Gently suggest to his mother that he seems to be having trouble adjusting to group play or whatever. See how she reacts. If she's open, suggest that she speak to her pediatrician and/or get some early intervention evaluations and services from the town/school system (free). If she's defensive, then just limit or eliminate play dates because of the hitting. Tell her, with compassion, that social get-togethers just aren't working now, and that you'd like to resume when he gets through this "phase." If she doesn't take care of it now, he'll definitely be referred for services when he goes to school. It will just be a lot harder because she's not addressing this at an early enough age. Meantime, he is clearly not a happy child, and hopefully she will want to help him.