In the state of Kansas how old does a child have to be to NOT choose to visit noncustodial parent?

I have custodial custody of my eight year old son who DOES NOT want to visit his father anymore due to his wife. This person is very mean and does and says things to him when his Dad is not around. He calls me several times during the weekend saying he wants to come home and why. Why do I have to make him visit those people if the wife is going to continue to be mean?