DS promised me he would not go back and forth to GM's when he was annoyed/upset and that he would talk to me. In fact, he insisted that he wanted nothing more to do with my mother. He said she had logged into his facebook, facebook friended many of his friends, rung some of them, gone through his bank statements and accused him of spending all of his money on alcohol. He said he didn't trust her at all. He also told my mother this, but she denied it, even though I know she did, because she told me how much he had withdrawn from the bank and I found her on his facebook one day. She also told me she had rung his friends crying, because she was so worried about him because he wouldn't tell her what he was doing or where he was going.
My mother had a habit of firing question after question at him, and even though I pointed this out to her when DS was not there, she still did it as soon as he showed up, while he was still living with her.
DS returned home, and asked me to go to a parents consultation at college with him, which I did. He said he was behind with his work. When we went, the tutors told me DS was wasting his time there, that he had been turning up to college every morning 'wasted'. I assumed he had been drunk, or suffering the after effects the following morning and asked my mother if she knew about this, but she denied all knowledge and said she had driven him to college every morning and never noticed he was drunk or hungover.
I later discovered DS had been taking mephedrone, and he told me he thought he had become addicted to it. 
DS worked hard to catch up at college, and kicked the drug habit. He eventually passed his course. 
DS had anxiety problems and severe depression, he was suicidal. He was also suffering from hallucinations when he was triyng to sleep. He had never been like this before, and I did the only thing I could, I persuaded him to go to the drs and get some help, which he did.
When he came back, my mother began dropping in to mine many times a week, she rang DS, rang his friends and poured her heart out to them. She has since denied she did this and continues to deny it.
DS would disappear to his room when she came round. Mother was extremely persistent in trying to get through to DS.
I had split with my partner by then, and DS seemed to prefer it being just me and him, although he wasn't too keen on my younger DC.
I worked hard to get DS to take responsibility for himself, I expected him to help in the house, to clear up his own mess and to arrange his own dr's appointments. He did all of this because he had to.
A year later, he left again to go back to my mothers. He found my younger DC difficult to live with, and my mother said he could stay there at the weekends, but of course, from the word go, he went and stayed permanently. I was concerned that all of the hard work into putting right the wrongs would be undone and asked my mother to ensure he continued to help in the house, contribute to the general running of the house because he desperately needed life skills if he was ever to live on his own, they would make his life easier. Of course, from day one, she just reverted back to how she had always been, letting his friends stay over, asking him for nothing, making his bed, treating him like a king.
She insisted on doing everything for him, even washing his hair for him FFS. I told her she wasn't doing him any favours, and that I found it odd behaviour tbh, but she said he worked all day and she didn't she is a pensioner and that was the way she was, she does the housework and has a meal on the table for the man and he works.
I did try to point out that DS was not financially supporting her though, but whatever I said fell on deaf ears.
So it went back how it had been, DS began drinking again, letting friends stay over and when I tried to intervene, mother would scream at me to leave DS alone, that I could not tell him to buck his ideas up in her home even though she was complaining constantly at how rude, lazy and disrespectful he was because apparently I was disrespecting her home by telling him off. 
After a year, he ping ponged back here again having fallen out with my mother again. I told DS this had to be the last time.
Mother was devastated again and told me she wished I would not put a roof over DS's head, she said she wanted to be the only person he had to go to, because she wanted to see him come crawling to her and have the satisfaction of saying No, after he had hurt her so much. She said she had felt threatened by DS and his friends and she would never let a man hurt her like that ever again.
She was talking about DS like they had been in a husband/wife relationship.
Almost a year passed, DS was now 21. One day we had an argument, and I asked DS to go for a walk, and think about how disrespectfully he was speaking to me, and to come back when he had thought about it.
He returned a few hours later with a van he had borrowed and proceeded to move his things out, no warning, no nothing. I was upset but held the door open while he loaded the van.
Mother was here when he turned up with the van and although he didn't go to mothers, she was as surprised as I was that he was going, she then did everything she could to find out where DS had gone, she offered help with anything he needed, was as nice as pie to him, and told him if he told her where he was, she would take in his washing and ironing for him. She invited him over for dinner a few days a week, and was always offering to drop things off to wherever DS was living.