I'm so sorry you're having to go through this OP and I have little practical advice to your awful circumstances. As a PP has said, the damage being caused by your Ex is also being inflicted on your Son, he's not mature enough yet to see through his Dad so he's parroting his behaviour because he's probably trying to show his worth to the parent that left. DH did this, it was different as his parents were as amicable as possible, but because he spent most of the time with his Mum, his Dad became almost saint like in his mind. He would take out all of his frustrations on his Mum but never question his Dad.
When we spoke about it recently he said that he thinks that because his Father was the parent who he had to travel especially to see, it scared him because if he didn't become the 'perfect son' to appease him, his Dad might leave for good. He didn't see the hurt he was inflicting on his Mum and that was in a situation where both parents tried to be fair.
As for his unsupervised access, have you considered taking him back to court with good representation on your side? I've never been through the court system as an adult (I did as a child but don't remember much of it). I know that there is funding for legal aid if you don't have enough to cover the costs of a solicitor:
childlawadvice.org.uk/information-pages/legal-aid-for-family-law-matters/
And I think the CAB offer advice, but I obviously don't know what avenues you have tried already.
You're doing so well and sound like a loving Mum to me, a loving Mum who dislikes the behaviour being implanted in her son's mind and who wants him to be free of this awful form of manipulation, for both your sakes.
I have read about how the courts minimise domestic violence in these cases and it's actually made me sick to see how womens rights for protection are still so backwards! I hope you can find a way to come through this OP and hopefully, after your Ex sees that it's not affecting you (it is of course but he should never have the satisfaction of knowing that) and that you're keeping on the high road in this situation, he'll run out of steam.
Have there ever been any instances of violence towards/in front of your DS? From what little I understand that is taken more seriously than the mother reporting abuse against herself (which is awful for women who are/have been through such a terrible ordeal). If there is a way to prove that your Exes vile behaviour is having impact on your DS to a point where it's negatively impacting HIS life (such as behaviours at school, grades, getting into fights or anything like that) maybe the courts would take that more seriously.
If they try to comment that 'it'll get better when the routine is set' as a fob off at least this time you can say, that it demonstrably not better now the routine is set, because his behaviour is getting worse the more he is exposed to his 'Father' and so you want access to be supervised?
I wish I could offer more, keep going OP you are so so strong, well done and keep the light on for the little boy you used to know, he's still in there and with the way you have said you handle the situation, he will know in time what your Ex has done.
It doesn't help much when you're in the thick of it, but I would suggest that your DS is acting this way because out of all of the confusion, you are his only true constant. So he is pushing you in all the wrong ways to see if you're going to leave too. It's almost like a self-destructive defence mechanism. His life changed, he can't see it from any other perspective other than that yet, so the one real 'safe space' he has with you, is what he is scared of losing the most. 