Unhappy grandson

My eight year old grandson and granddaughter have to go to their dads every other week on a Friday and come back the next day. My grandson dreads it and it really has an affect on him. His dad doesn't help matters as he never spends any time with him when he's there. My granddaughter doesn't like going, but puts up with it. Also the dad and his wife have three daughters who she doesn't mind playing with. My grandson has tummy ache and begs his mum not to make him go every time which upsets my daughter. Can a boy of eight decide that he doesn't want to go to his dad's?