violent/aggressive behavior in 3-4 year old

I am very concerned about my cousin's 3 year old son.  For more than a year (closer to two), this boy has been very aggressive and violent in social settings.  My children, or anybody else's children,  get smacked, scratched, or hit everytime the kids are together.  This behavior has become more intense over the last 6 months.  Everybody, including the parents, attribute this behavior to "well, he's bad, just a toddler..", or "his older brother and sister smack him around so he learned it from them"  My children understand that he is smaller and they are not to hit back, but it is frustrating to have your child constantly beat up.  I have noticed that he does not smile like a normal kid when he is playing and will not return a smile- he always looks angry.  Sometimes I call his name and ask if he wants to sit on my lap and he will scream, "NO!"  "SHUT UP!"  The parents will, at all costs, try to keep this kid from freaking out - even when he is wrong!  But his tantrums are so bad, nobody can take it.  It has gotten to the point where i do not want my 5 year old around him because I am concerned about my kid getting hit in the face every time. I have noticed that the mother is becoming isolated.  For example, she can't find hardly anybody to baby sit her kids if the 3 year old is part of the package-her own family refuses at this point.  Every week while her daughter takes dance class, she is no longer bringing the 3 year old with her because his behavior is so bad while they wait.  This past week, he ran from her and pulled the fire alarm...  The mother mentioned that this Thanksgiving they would be spending at their own home rather than "dragging" the kids to the big family get together an hour away.  I know it is because they are becoming increasingly embarrassed about his behavior and do not have the willpower to put up with his tantrums in front of the whole family.  As far as developmental delays, I believe he demonstrates some issues with speech.  His motor skills seem ok (he can pack quite a punch) but he is still not fully potty trained.  He also has a very short stature.  Any thoughts on this would be greatly appreciated.

[Encephalomalcia]

He needs to stop this behavior and his parents need to take charge and control of the situation.  When you hits someone he should be told in a very loud deep voice that he is not to hit anyone.  If he starts school and is still hitting he will meet another little kid that will really hit him back and possibly hurt him.  The potty training is no big deal.  Lots of kids are not trained until well after 3.  When he is ready he will go, but a child cannot be forced or they will rebel.  He will probably need speech therapy if he is not speaking clearly by now.  Bottom line is the parents need to start being parents.  Firm when he hits.

[margypops]

I agree with the firmness , however also take a look behind the behavior and check out if this child is upset by anything or anyone,this does sound like a child/parent interaction problem, there is a good book out there called SOS Help for parents by Lynn Clark,