Whats wrong with him?Is he sick or just bad attitude?

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To all of my fellow mylotters, Hi,i want to have some advice regarding what should i do with my youngest sibling. Everyday he is happy whenever he will make some stupid things to make my kids angry and fight with him but when he get hurt he will fight back with the kids. Whenever he sees everybody is silent he will do thing to make noise.He loves noise. He is very hyperactive as he wants always teasing and making stupid things to my kids and all the other children around him. I feel like he hates piece and silence because he loves always making noise and teasing and make the kids cry but when you scold him he will fight back and dont accept his mistakes. He is already 14 years old and he thinks and acts like small kids. I dont know what to do wit him..