By law, your child has to go. No matter how unwilling he or she is.
The only way you can stop the process is to gather enough evidence of him abusing your child.
From your great wall of text, I see none.
I only see your hesitation in persuading your child to go to his father. Which is a grave mistake.
Your boy is 6 years old, he can understand if you teach him why he has to go to his father.
Reason: If he doesn't understand, why would he want to go? He is throwing temper as he is unhappy, that's it.
You fail to see that he do not understand it's a right thing to do and you didn't teach him. It's your responsibility.
If you dont't understand why you must do something, you won't want to do it. This applies to everyone and anyone.