I am a 65 year old grandmother with a 4 year old grand son,

I am a 65 year old grandmother with a 4 year old grand son, born with hydrocephalus and cerebral palsy. His disabilities are in his eyesight and hearing and left hand spasticity.
There has been a lot of distress between my son and his partner, with his partner losing
her temper and acting very angrily towards my son, so much so that he separated with her a year ago, but has supported her financially and emotionally through this time.
My grandson spends time with his father twice a week, from 3.30pm to 8.30am in the middle of the week and then at the weekend from 4.30pm to 4.30pm the next day and during this time I am able to spend some time with him.
He is a very good and bright little boy, but we have noticed that when he is playing or enjoying himself he does not like to be disturbed and has to be encouraged to interact with others. This has caused a huge problem with his mother, who takes this rejection very personally, and loses her temper and screams abuse to her son, until he is in tears.
This has happened on numerous occasions, mostly when she is collecting him from my son's home. This morning it happened again, my son had been playing with his son, they were laughing and happy, waiting for his mother to pick him up. When she screeched to a halt and started screaming to the child 'If you can't speak to me on the phone to say goodnight, don't speak to me at all' with a lot of extra screaming in her language that only the boy understands, she dragged him into the car and drove off. My son, was talking to him an hour before the evening call saying that they would watch the end of the movie, have a bath and then phone mummy to say goodnight, trying to prepare him, however, during the bath he said he didn't want to talk to mummy, and just didn't want to say goodnight and gave the phone back to my son. I believe that her reaction to this is mental abuse, and we are all in shock and do not know where to turn. Where do we go?