I can relate to that Durhamjen. My mother in law became docile and vague in her dementia in her 80's and 90's but she was not ever thus. As I mentioned in another thread, she lost her first child, a little girl, to pneumonia at less than 3 years old and never really got over it.
Her frequent breakdowns and hospital stays were so distressing - one extreme episode led to paranoid delusions where she accused me of all sorts of monstrous things. I had apparently aborted the twins I was carrying, I had arranged for my son, then about 8, to have both his legs amputated - she became so agitated over this that we were forced to make him take his trousers down to prove he still had his legs. That was the final nail for me and I wouldn't let the children see her again until she had come through this awful time.
So yes, I realise mental illness can take many forms. So sad for your mum and grandson not to be able to share her last months together happily. I hope he has some good memories of her, memories of the real person she was.