Why is my autistic brother acting like this?

well my little brother is autistic and he is now 6 years but still acts like a 2 years. He yells at his parents and myself for no reason, he will say no to anything and he will cry like a baby loud when you try to make him do it , he will cry so damn loud that the neighbors think we are abusing him. he still urinate on himself and sometime poop on himself. he will yell at you if you try to change his clothes or help . he doesn't have any kind of respect for no one in the house to the point that now the house that was once extremely family bonded and joyful is kind of depress. i am a 16 years old and when my other friend tell me what they are able to do with their little brothers, i am not feeling bad for me i am actually feeling bad for him. we already tried therapy and he is in a special class. we all don't know what to do when he cries for small stuff, letting him be alone doesn't work, time out doesnt and even bribery doesnt work. he is a nice kid once a week, but most of the time he act like bad kid. we came to the point now that we tell him that whatever he is crying , he is right we are wrong or we'll tell him we are on his side but he yell even louder so that whatever you saying he is not hearing. help help help anyone ever heard or knows a case like this one