K.F.
I would call in the authorities on your parents. Family is important but both of your parents have a responsibility to care for him. The state has money and resources to care for children in your brother's situation. He is 17 and this is a very hard age. Not quite a boy but definitely not a man. Should you decide to take him in, your boyfriend and your brother are going to have run ins because of the dynamics of things. It may not happen but I anticipate it will. Since your father is abusive, your brother may have learned the same coping techniques or the lack there of.
I would definitely get the state involved because then the parents can legally sever the responsibility of your brother, there would be a stipend to care for your brother and you could still provide a safe place for him to land as he tries to soar to adulthood.
Just some food for thought. I hope this helps.