My 8 Year Old Grandson Has Sensory Perception Disorder. He

My 8 year old grandson has sensory perception disorder. He sneaks around intentionally hurting his 5 year old brother. His parents ignore the situation. He is twice the size of his brother. He definitely has a learning disability but his parents do not discipline him. He acts and talks like a 3 to 4 year old. Has no manners. Cannot answer any questions asked to him. He clings to his mother and sits on her lap. He is 4' 11" tall which makes him appear more disabled. His mother coddles him and allows him to act like a baby. Because of this his little brother is ignored. What should be done about this situation.