My attitude toward potty training is not very popular, but here it is. I didn't care if they were not potty trained until they were 18. At 18, legally my job is done. If they can find a girlfriend, she can help them. In the meantime, they can carry around a jaunty little diaper bag.
Both of my sons were late to potty train. They were both pooped trained about 9 months before they were pee trained.
One day both of my boys just decided they wanted to wear big boy undies and use the potty. There was no magic wand or toy.
Bottom line, you can't control what goes in them or what comes out of them!
As far as your daughter, her attitude is most likely "rubbing off" on your grandson and making it harder for him.
What is the rush? Is your grandson enrolled in preschool in the fall? Preschools cannot legally bar him from attending b/c he's not potty trained. The law states they must make "reasonable accomodations." Reasonable accomodations are that if he completelly soaks himself and/or poops, someone (a person your daughter designates such as herself, husband and/or you) must be available to come to school and change him.
Holding his bowel movements for two or three days can lead to encopresis. Children have fewer bowel movements than normal, and the bowel movements they do have can be hard, dry, and difficult to pass. Once a child becomes constipated, a vicious cycle can develop. The child may avoid using the bathroom to avoid discomfort. Stool can become impacted (packed into the rectum and large intestine) and unable to move forward. The rectum and intestine become enlarged due to the hard, impacted stool. Eventually, the rectum and intestine have problems sensing the presence of stool, and the anal sphincter (the muscle at the end of the digestive tract that helps hold stool in) loses its strength. Liquid stool can start to leak around the hard, dry, impacted stool, soiling a child's clothing.
Good luck to your grandson!