I don't know how old you are, but it sounds to me like you need to move out ASAP. Your grandparents either can't or won't change status quo - if their son uses them it's because they permit it. There's certainly no reason for their loser son to change his life. So, all you can control are your actions.
Save up your cash and rent a room somewhere, and in the meantime secure any valuables that you have in the house so the druggie doesn't steal your stuff to pay for his habit.