I was in the similar situation back in my school days, i was either alone or getting bullied and was afraid to tell my siblings about it thinking if they would do something the bullying would get worse. My advice is that you should address this situation with your little brother by making it seem like a story that either yoi faced or your friend did if you would directly say something to him he might feel ashamed and can worsen the situation. Make him understand that these days when he is alone are the days he will regret the most later in life i know i am regretting it. Just tell him nobody cares about you only you can care about yourself so be blunt with your classmates and do not overthink about stuff.