He made a three year-old sleep in a tent by himself? Wow. This is such a heartbreakingly difficult situation Scout. I am so sorry you are going through this. I’m with you… I’d feel better having OW there too as it sounds like your XH is a wild card.
Your son certainly sounds like he is being impacted by his dad’s parenting. His sudden reaction to correction after just coming back from a visit is a bit of a red flag that you are going to want to pay attention to.
You’re right. At this stage, there is not a lot you can do to change what XH is doing. He does not sound like a co parent…more of a dictator.
If I were you, I would consult with your lawyer and ask his/her advice. I would also document absolutely everything. Keep a journal of dates, your son’s affect when leaving and when returning, anything he says about what happened, who else was there, etc… Chronological notes are very difficult to dispute when they are done consistently over time. I had an emotionally abusive supervisor who had it in for me. At the suggestion of a coworker, I documented absolutely everything he said and did for about six months. He eventually fired me (totally bogus reason) and I sent in a copy of all my notes to his boss. He was fired two weeks later and my coworker heard his boss say to him that he was the worst supervisor he had ever heard of. Different situation, I know, but I never would have gotten anywhere without those notes. They will also help if you ever find yourself in a situation of needing to make a report to CPS.
The other thing I would do is find a good therapist for your son. At his age, you want to find someone who does play therapy. It’s not going to change his dad but it may help him to cope better. (((HUGS)))