No one's fault here. You are right at your end, he is right at his position and his mother is also. She seems the right cause at her age and with some anti depressant tablets it's understandable that she will behave unexpectedly. Her son is right cause he is a son and should take care of her at this time irrespective of her behavior. You are right cause you did all it takes and continuously tried to be with him and save this marriage.
Now the only point is, is there a way to settle this all down. I mean if you can adjust with his mother, he can pay attention to you as well. Is it possible for you to ignore her mother's behavior and just do whatever it takes to keep it neutral maybe like just listen to her and no need to respond to anything which you may not like. Not easy but I hope you understand. Is his son can manage it between you and his mother considering both are important parts of his life and this is the time which requires most patience, tolerance and calmly execution of every behavioural activity.
I am not in your situation so I can't understand it exactly, I know advising others is easy and it seems hard to work in your own life. But believe me little tolerance and ignorance to things which may cause noise or fall you apart will keep this bond life time.
Just understand you and your husband are the only ones who can solve it, no one else can. Separation is easy but as you mentioned he was good so finding a good guy is not easy.
Wish you all the best and prayers to God to give you strength to sort this out and again settle with love, joy and happiness. God bless you.