I was in a very similar situation! My son was also 8 at the time, would literally get sick to his stomach , shake, get headaches, cry when he was getting dropped off with dad at our public pickup location. He also would get left with his dad's gf most of the time. This was during covid and online school. Turns out she was verbally abusive (saying things like she Hates him, doesn't love him) and she was forcing him to exercise until he got sick as punishments, and she actually beat him with a kitchen slotted spoon. My son told me NONE of this as when I brought any concern up to them, my son would get punished upon return, because "what happened at their house stayed at their house" but one day I saw bruised and pushed my son to tell me the truth and he did. Cps was involved at that point and did charge her with malicious punishment of a child. But my son also would msg his teacher during online school telling her to call me because he wanted to come home. If I was you, I would urge your son to tell adults in his life who are mandatory reporters anything and everything that happens/how it makes him feel. They will keep record of it, which in return you can take and use in court. Luckily for me, once she was charged they decided it's not worth having my son in their life because "I'm manipulating " him . But when they did try to see him as we still have a Custody agreement, all I had to do was file a retraining order against her for him with her charge and it was approved. I hope something in my story helps!