My God!!! This sounds so familiar, apart from in my case, it is my mum and DS.
Mum doted on him to the exclusion of her other GC since he was born, and as he grew older, he ran to my mums every time he didn't like what I asked of him. He would wait until I had to go out, then ring my mum, and she'd roll up before I returned and whisk him off to hers for a few days to give me a break apparently.
Back and forth, back and forth, until I felt I couldn't leave the house. I asked my mum not to pick him up, yet she refused to stand by and do nothing if he needed her, I couldn't ground him or punish him because he would leave to go to my mums.
Eventually, he left to go to my mum's permanently when he was 16. Police did nothing.
Since he went to my mother's permanently, she has waited on him hand and foot..in return, he has absolutely no respect for her, but tbh, she is all hot air....moan moan moan...nag nag nag....nothing changes.
I am sick of hearing it, and have now told my mother I don't know what the answer to DS's lack of respect to her is, or why he doesn't give a toss when she pulls her hurt face (Can't he see I'm suffering?' she says).
The truth is the answer is not what she wants to hear though. Stop issuing threats you have no intention of carrying out, they have lost their effect, and he knows you say them in anger...start telling him what you will do if he does XYZ and then DO IT if he actually does XYZ IYSWIM.
Just one example....DS arrived back at mums after a night out with 2 complete strangers and told (yes told, not asked) mum they were staying, because they had nowhere else to go that night. (Too far to travel back home and not enough money for a taxi).
Mum said she was frightened.
Mum could have told DS 'Get them out of my home in 5 minutes or I will call the police and have them removed' and do it if they don't leave.
What did mum do?? She hid in her bedroom clutching the phone and cried and panicked all night unable to sleep, yet she didn't phone me or my sister or anyone who would've come over and turfed them out. 
She went mad at DS, who took very little notice, and told him if it happened again, if he brought friends back that she didn't know, they would not be staying again. She would not live like that again.
GUESS WHAT HAPPENED LAST WEEKEND?
Yes, DS invited another 2 friends mum has never seen before, who came into her home, swearing and shouting...left oily footprints all over her carpet....and what did mum do??
She let them stay (because of course they had nowhere to go that night again) and got on her hands and knees to scrub the carpet the next day, after cooking them a full english breakfast first. 
Apparently, she has had yet another go at DS and told him it definitely wont be happening again, like she has told him she is kicking him out umpteen times and never ever does.
I wont hold my breath while I am waiting, and I will be surprised if DS ever shows her any respect.
I blame her for allowing him to treat her that way. Just throw him out.