Just a few thoughts of mine, not sure how helpful they will but it relates to some of my own history.
I'm sure its a difficult situation all around and I know you probably have mixed signals from different experts on how to tackle the challenges there. Personally, I don't think there is anything wrong with discipline and you should not at all tolerate that type of disrespect. However, you shouldn't be engaging in it either, not by arguing. You have a place in your sons life and that wont always be pleasant nor will you be his friend. I was listening to an audio book by Jordan Peterson and he had a section on this. It went something like "Do not let your children do anything that makes you dislike them". He mentions how as parents one of our purposes is to teach our kids appropriate behavior in society so they can turn into healthy adults. He briefly goes into using the minimum amount of force (or correction maybe) to teach a kid who is acting badly how the behavior is not appropriate. For some kids that means having something taken away as a punishment, for others its a time out, others may need some physical correction (smack on the hand up to spanking, even slight restraint). I know much of this is controversial (spanking for instance), but personally the way he laid it makes complete sense and I honestly agree with it. What I really took away from the chapter was that we can either teach our kids appropriate ways to handle it now, or society will later. And when society does it, it will be a much more difficult and harsh result. In the short term every time you allow a kid to do something like that, you allow a bit more resentment to form, which will only contribute to things as time goes by.
Another point is, I don't think discipline with a child and validation need to be a one or the other choice. You can have both and even use them at the same time. its ok for a child to be upset that he has a toy taken away and you can even validate their emotions. No emotions are wrong, but actions from them can be. However, you can also be firm and tell them why they have the toy taken away and that you will not tolerate that type of behavior. I know its not as easy as it sounds, but from my own perspective it works. Honestly the most difficult part is me getting my own emotions into check so I can avoid overreacting to them in the moment. You are the parent, there is no arguing with you or disrespect, and if there is, there needs to be consequences to it.
My son was crying yesterday and furious with me just before because he lost his tablet privileges and was "bored". He wasn't listening to me the day before or that morning and I got fed up and took it away. He has this habit lately with arguing with me on everything. I know the circumstances and behavior aren't the same so the behavior may be different. My son will drive me insane by not listening. I can literary tell him something and he goes off into his own world and will walk the opposite direction of what I just told him to do. I don't even think he does it on purpose, he just instantly loses his focus, but it absolutely infuriates me. There isn't much of the rebellious behavior right now, I usually shut that down direct disrespect very quickly. Yelling and getting super angry didnt work, but being calm and enforcing boundaries and consequences usually works. I get some of it from my daughter rarely but I just stand my ground and it passes after a big. For me its important to talk with them when both of us are calm and explain actions and consequences and even validate their feelings around it.
Another part I wanted to touch on was the reasons for your divorce. I don't see explaining to him the reasons or bad talking your ex as productive at all. This is a more extreme example but when I was little both my parents would talk badly about the other. Most of this talking was over the reasons they split. It involved drugs, cheating and other insults that may or may not be true. The only think it ever did was make me feel like [censored] and make me resent them. To the point I nearly disowned them when I was 18. When my mother would say anything about my father, I would take it as personally toward me, even if she never intended that that way. Same when he would talk to me. Looking back I see in the moment they were just trying to relieve themselves of pain and explain why our lives were in this broken family, but it never did anything good for me. Anything negative you say about your ex will only drive a wedge between you and your son. That's how I avoid talking about it to my kids, I know the long term damage it does. At this point when my kids ask me anything about it I'm honest to a degree, but they don't need specific details. "Me and your mother are divorced because that's the decision she wanted to make, I had no say in it. Past that, we both may have had issues in the marriage, but she choose to end it". that being said, again, I don't think you should tolerate his anger toward you. HOWEVER, do not argue with it and try to make your point. That will only backfire in the long run. I would find a way to say something like "He left, his choice. Why our marriage was in the condition it was in is more complicated and each side may have their own perspectives". You've mentioned in the past he knows how to get under your skin and its likely what hes trying to do here. You have to be the one to break that back and forth cycle and stop feeding into it. Stop reacting to it. I know that's not easy, but you can do it.
Also, looking back no matter what my parents said, I formed my own opinion on what happened between on and much of that is based on their actions. The parent who tried to "explain how bad the other is" usually took more of the fault in my mind. The best possible thing you can do is let go of those reasons. Some things will come out in time, but you don't need to say it, and trying to only backfires on you.
I'm not sure if any of that really helps but I thought I'd share.
Accept what is, let go of what was, and have faith in what will be
