Violent little brother?

My little brother is nine, and gets very angry when he doesn't get what he wants (the computer) sometimes he clenches his teeth and puts his fist under my chin to threaten me! He also gets veryy emotional if he gets in trouble for something he did, he has the symptoms of adhd but my mom doesn't want to hurt his feelings so she says he doesnt,. He can be really sweet if he wants to be but if he doesn't get his way he blows his top! The other day he punched me when my mom wasn't around just because I was on the computer and he wanted it, my mom never believes me when I try to tell her, what should I do?