Co parenting drama (advice needed)

Long story short, my partner & myself do not get along with his ex for numerous reasons.

My partner has certain days with his son that him and his ex agreed on whenever they had went to court.

My partner was trying to compromise with her by asking if he could have his son for one entire week and she sees him on Wednesday and then when it’s her turn she has him for an entire week but he sees him on Wednesday.

Because otherwise his son would’ve had to spend an entire month with his dad and then an entire month with his mom during the summer break. But he’s very attached to his mother.

Today is Wednesday and my partner was supposed to see his son as agreed. She has her 8 year old son call his dad upset saying he doesn’t want to come to our house. His father calmly tells him it’ll only be for a day and that he will see his mother tomorrow and he begins to tear up and say he just wants to stay with his mom. This happens every time he goes to his moms and then it’s time to come back to his dads. So his mother takes the phone and continuously says that she doesn’t know what’s going on in our house and doesn’t get why he doesn’t want to come over but that his son can see him next Wednesday. Which means he would go an entire week without seeing his son. She’s a very difficult person to deal with. Very immature for her age. She just kept implying that our home is not safe.

Now here’s what is ACTUALLY going on. At his mothers house his son is spoiled. He always sleeps with someone because he doesn’t have his own room there. They baby him, he has no bed time or rules, no chores, they basically let him do whatever he wants. Especially on that his grandmother is at his mothers house for a while.

At our home he has his own room, we ask him to wash some dishes and to please clean up his room, he has a 10pm bed time, and the tv has a timer on it because he tends to stay up till 2am watching tv.

When he’s at our house at first he’s upset because he wants his mom and grandma. Doesn’t help that his grandmother and mother makes it seem like he’s replaced by his new brother, or make it seem like his grandmother is going back to the state she lives in. But after a day he’s fine and doesn’t want to go back home. But then once again when he’s at his moms and it’s time to come back it’s a constant battle.

When he’s here I try to do everything for my son so his father can focus on him. Which he does. I don’t know what to do or how to help my partner. I see it hurts my partners feelings when his oldest son doesn’t want to come over and chooses his mom over him every single time. Anyone have advice? Anyone been in a similar situation? What do I do? How can I help?