Should I force my 13 year old son to go on access visits to his father's house? There is no abuse happening?

But my son tells me he is miserable there. That my ex only dumps him in front of the TV or sends him off to a friends house and spends all his time with his new girlfriend....and that the only time his father ever speaks to him is to criticise him or interrogate him about my movements and who I have been seeing..... show more But my son tells me he is miserable there. That my ex only dumps him in front of the TV or sends him off to a friends house and spends all his time with his new girlfriend....and that the only time his father ever speaks to him is to criticise him or interrogate him about my movements and who I have been seeing.. He says he is miserable when he is there and feels unwanted. I have been forcing the issue of his going because I felt it was important he maintain contact, but he is getting very stressed now before each visit and told me this time that he had felt like he wanted to vomit all day.

He and his sister left about three hours ago and I have been feeling guilty and worried ever since.
His sister says similar things but seems to deal with it better.

What should I do??
Update: I've tried speaking to my ex. He yelled at me for about an hour, called the kids liars and told them that all they wanted was for him to spend money on them. He is a real douche, though I try not to say that in front of the kids.
Update 2: Ummm...(so humiliating) the girlfriend thing is a bit more complex that usual........she was my best friend for almost 15 years....until about 3 years ago when she broke with me...which coincidentally would appear to be about the same time she began sleeping with him.
Yeah...tawdry...I know.
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