With my stubborn daughter, I waited until I knew she had to go (she got so she could hold it for hours) and then when she came home from school I told her that I couldn't have pee on the carpets so she was going to have to stay in the kitchen. We had baby gates so she was pretty much stuck. We had a TV in there, so she could watch it, but it wasn't comfortable. It took 5 minutes and she said in a defiant tone, "Fine, then I go!" She stormed into the bathroom, grabbed her portable potty, took it into the kitchen, slammed it down, sat on it and went. We cheered, she called daddy and grandma and got a reward. Pee wise she was good to go after that. Pooping was more of a challenge. The doctor said to sneak fiber into her diet as much as I could so she would have to go. We started putting the potty chair in our living room in front of the TV. Gross but effective. I had to make up a silly story about sending her poop to a party in the toilet bowl. I also had to invest in the rewards that she wanted. She wanted tastycakes. She picked several kinds out at the store, called her potty treats. After a hostess ho ho she was trained.
My son was a bit more of a challenge. I had to take his diapers away and clean him up multiple times. When I knew he had to go, I held him on the potty while he cried (sounds mean, but I really think they have to understand the feeling). Once he went, it was like a light bulb went off. Oh, that was what I had to do. he called his daddy at work, and we went to the store to pick up the prize he wanted. It took a while to find out what he wanted as a reward too. His was a giant candy jar of candy he could pick from. And you have to find out if there is a fear involved.
Now my nephew took the cake. He was well over 4 and refused to go. My SIL was pulling her hair out. She left him with me over the weekend. I took him aside and told him that I really couldn't clean up pee and poop on the floor and he would need to use the toilet in my house. He shrugged and said okay. Every 2 hours I would take him to the potty and tell him to go. And you know what? He went. I told my SIL and she was floored. He still wouldn't go for her. A few weeks later, when they came to our pool, he clearly had to poop. She said she didn't care if he went in the pool. I very much did, as they have to shut the pool down for 8 hours if a kid poops in it. I took him aside and said, "if you go in the pool they will kick us out and we can't come back. Either you go in the bathroom or I can take you to my house, but either way you can't get back in the pool. If you go you get a treat at the snack bar." It was the first time he went for his mom. He got his treat and was trained. Grandma, you will probably have more success with your grandson than his parents will, because they are his parents and he doesn't want to give them the satisfaction. Just firmly tell him that he has to use the bathroom. And put a diaper on for naps. We kept them on at nap time and bedtime for a year after training.