I agree with some of the posters--heavy metal toxicity will cause all of your nephew's symptoms, as well as any food allergies or intolerances.
My daughter is on the Autism Spectrum; after I started buying fresh, organic produce for the family because of my own health problems, we noticed a remarkable change in my daughter. Her anger and agression disappeared, she stopped hitting herself or banging her head against the wall. By buying organic and cutting out processed food, we took away pesticides, chemicals, additives, colorings, etc. By deleting cow's milk from our diet (my hubby and I are casein allergic) and using goat and sheep products, rice milk, coconut products, etc., cutting out gluten (wheat, rye, barley) and soy, and generally, detoxifying her body, it made a tremendous difference. I also had my daughter tested for heavy metals after I tested severe myself. She chelated 3x; that did a world of good.
Your sister needs to take your nephew to a Naturopathic Doctor; allopathic doctors DO NOT test or treat for any of these problems (heavy metals, food allergies, food intolerances). If she wants her son to be better, she has to take him to a Naturopathic Doctor and help him herself.
No child should ever be that angry!! From what you describe, your nephew has something severely imbalanced. If it's not fixed now, it will ONLY get worse. Can you imagine trying to control him when he gets bigger and stronger? I pray your sister will listen to you and get her son the help he's desparately screaming for. There won't be a school anywhere who will allow him to act like that in the classroom or out on the playground. In fact, I can guarantee you that the school will medicate him in order to keep him in line. And boy, giving prescription drugs to a child; you're just asking for even MORE problems down the road. One child we knew who was on Ritalin developed diabetes at the age of 10 from taking the med and now has to take insulin daily.
Please, try to get your sister to take your nephew to a Naturopathic Doctor and get him tested. Your nephew will thank you.