First of all...DONT BLAME YOURSELF! If you have any relatives that live nearby than go live with them. I know it is hard to leave your 'little brothers' behind, but he is affecting you and your mum. You BOTH need to get out of there. Ask your bf is you could stay at his place for a little while. You and your mum. If not go and stay at a near by hotel. It isn't your fault and you shouldn't blame yourself for it. What he did was HIS fault and he took advantage of YOU...Sadly there are sick people out there who do this.
I suggest that even though there isn't any evidence saying that he did what he did, you should go see the police. If all else fails, I'm sorry, but that is all I can think of.