Potty training

My nephew is 6, he didn’t go to school this past year . So he has to go in the 22-23 school year. He didn’t go to school because he’s not potty trained yet . Any tips or suggestions I can give my sister in law to help make the process quicker so that he can start school in august ? I suggested a sticker chart, she said that worked for getting him to pee but he will only poop in a diaper.