IMO, we don't know what is going on over at Dads. My 7yo went through this stage as well. Personally, I did not like forcing him to go, but his father started to threaten to take me back to court to change custody if this didn't change. He was convinced I was responsible for this. I already had son in individual and group counseling. And I knew that there was a lot of drinking going on over there. Son does not like to hear his father retching all hours of the night. He was pretty much sitting on the couch over there all weekend while Dad and his SO drank all night.
I didn't like it, but I knew I could only do so much and after that, Dad still has his rights. Son would run and hide when he came to pick him up.
I got tired of the fight.
Telling son that there was a chance judge would make him live with Dad was the only thing that got through to him. He realized that sitting on a couch every other weekend, watching Dad drink for 2 days striaight was humonogously more appealing than living there full time. Now he goes, plays his PSP, sits on the couch and watches tv, goes to bed when he is told and Sunday evening he is very glad to come back to a normal home.