Sorry for all you're going through. Please be very careful where your mother is concerned. I think, for her own protection, it is important to explain what is going on. I had to do this with my 84 year old mom. She was constantly giving my son money to do work for her; somehow the work never got done. She is in early dementia and is easily taken advantage of. I have had to hide her valuables, bank account info, credit cards, checks etc., from him. If your son is not stealing yet, please consider the possibility of this, and protect your things as well. I know it's hard. I never expected that my son could do the things that he has done.
And don't worry if he threatens that you will never see him or hear from him again - he is just trying to make you feel bad for setting your own boundaries. You are on the right track - stay strong and continue!